Also output the URL of the original exercise image file
Allow ordering of endpoints

For this to work, it is necessary that the ViewSets have either a
'ordering_fields' or a 'serializer_class' attribute
Add filtering of endpoints

This needed an external django app, 'django-filter'
Update and refactor the testcase for the REST API

Since the API now supports write operations, the testcases test
performing DELETE, POST, PATCH and PUT operations

--HG--
rename : wger/config/tests/test_language.py => wger/core/tests/test_language.py
Fix active tab when browsing the api
Update the documentation for the new REST API