Explicitly reset services between tests

This adds a new test support script which resets the Collections API 
service between tests. This is so that, when we override the Collections
API with a stub or mock, the mocks do not leak into other tests which
rely on the standard service behaviour.
Expose 'start' and 'total' values from API response

This changes the Subcategory model to expose the 'documents_total' and
'documents_start' keys from the Collections API response as instance
methods.