Allow cross-domain requests to be handled by the web service
Support Accept header properly, fall back to JSON when responding
Add JSONObjectConverter to add support for to_json() class methods

This will allow us to automatically render objects to JSON. The
to_json() methods are expected to return a Python data structure
that the default JSON encoder can handle.
Make objects, properties, timestamps, references JSON-serialisable
Squashme: Add missing unit tests for to_json() methods
Always run test suites for different APIs in the same order
Make property definitions JSON-serialisable and update unit tests
Make class definitions JSON-serialisable and update unit tests
Make schemas JSON-serialisable and update unit tests
Make commits/refs JSON-serialisable and update unit tests
Make object classes JSON-serialisable and update unit tests
Add support for testing the web service with JSON and YAML
Merge branch 'jannispohlmann/json-web-service-rebase'

Signed-off-by: Jannis Pohlmann <jannis.pohlmann@codethink.co.uk>
Reviewed-by: Adam Coldrick <adam.coldrick@codethink.co.uk>