To create "tagged" items with terms
to add tags to items via the api

We don't want to see this, the name and taxonomy should be enough to
idenfiy them.

Had to make the nested serializer writable to stop other tests failing
(and because I think we need this later for creating tagged items on
import)
Also moved some meta classes up to top of classes
to make them easier to parse.

Add passingt est for adding a Term to an Item
Add unicode methods for Taxonomy and Term
Change the unique contstraints for Term so that
Term name needs be unique only for a given Taxonomy

Removed taxonomy and term classes from data_layer - just reference the ones
in taxonomies app directly for now.

Will need to run "deploy/tasks.py clean_db" and "deploy/tasks.py update_db"

The last error from running the tests was that the TErm table didnt
exist in the test database. So I'm running

    manage.py makemigrations

And it fails with

    AttributeError: 'NoneType' object has no attribute 'unique'

Following the [django docs](https://docs.djangoproject.com/en/1.8/topics/db/models/#abstract-related-name)
I put in:

        related_name="%(app_label)s_%(class)s_term"

But still can't even get a helpful error message.

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*

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

These are no longer used by transport, and nobody else should be using
them.

By default the router derives the url names from
the model name (which it gets from the queryset)
which is Message

Note: transport still uses the explicit views

tests broken

byepassing data_layer.handlers, but I think they are
getting in the way of DRF goodness now.