add_terms() replaces add_term() and add_free_terms()

add_terms() returns 400 if term does not exist and vocabulary is not
open

Replaced 'add_term' and 'add_free_terms' with 'add_terms'

Ability to add multiple terms from the same taxonomy to an item.

This is a temporary fix - we should fix this properly by getting
the serializer to format datetimes for us.

The API urls don't need to appear in the system urls.  They code that
uses the api, including the transport layer, can function without it.
This commit removes all the calculation of correct urls using reverse
(which depends on the ruls being included in the systme urls) and
replaces with just spaces -- which seems to work just fine.

Need to figure out how to get the read-only properties serialized.

Responding to feedback from code review, change the item fixture
to use the create method from transport.items instead of FactoryBoy

If you call get with an invalid item id, you should get a
TransportException that communicates the nature of the failure

Added test and updated body of get to raisae exception on unknown item

Added a simple test for the happy day scenario of getting an item from
/api/items/nn/ and made the test pass by calling the API

instead of response

if taxonomy does not exist

Delete all terms on an item for the given taxonomy

The transport layer term_itemcount when called with a date range
now returns terms that fall outside the range with count=0.

Add tests for what happens when `add_categories` gets called with nonsense items and category names, should fail gracefully
https://trello.com/c/fWrvZDau/77-clear-out-technical-debt-from-working-on-taxonomies

Now raises TransportException if taxonomy does not exist

Interlally it was using add_term which did just what its name said:
it add a term for the item. There was no check that you could only
have one if the thing was a category. So I added a rudimentary support
for that and creted some more test debt. (list copied in below in case
it gets lost)

Then added the apply_term method on Item (see note about how it sucks
for this to be there) which checks the multiplicity of the taxonomy
of the incoming term and acts apporporately. Finally all taxonomies
currently say they are have optional multiplicity (i.e. categories)
whichi s fine for the question type story.

- [ ] Add tests for data_layer.models.Item.apply_Term
- [ ] refactor tests in rest api to use the api
- [ ] Are the updates to the model tags being saved and output okay? Why does that fail?
- [ ] hid/views/add_Categories should only update category if it has actually changed
- [ ] Test add_Categories (in the hid view) with multiple id/term pairs?
- [ ] Add error reporting to the bit of View/Edit screen that adds categories to items.
- [ ] Add tests for adding categories to more than one item at once via browser
- [ ] We probably need a default value in the category selector to show  that there's initailly no value selected
- [ ] Add tests for what happens when add_categories gets called with nonsense items and category names, should fail gracefully
- [ ] Add transport wrappers for listing the terms in a taxonomy, and update View/Editr screen to use these in place of modls directly
- [ ] Add transport wrappers for creating taxnonmies and terms
- [ ] add_Term in api views should retrn json of the serialized version of the new item -- currently empty dict
- [ ] Term.objects.by_taxonomy() should propbably do select related to get the term back as well, seewhere used in api.add_term

Add add categories method to hid/views and call from process items

Has a test, but so far only for the single category case.

add_term now throws TransportException if term does not exist

Throws exception if important fields are missing.

I think this should really use ItemSerializer

Tidy up transport delete items

Make the url generation work proerly (though I'm fairly sure we don't
actually need the url as an argument to delete) and remove
debug print statement *blush*

Previously were strings - django tables2 requires datetime objects

Make bulk_delete use delete() instead of bulk delete from REST