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.

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"