Backend tests

All serialisers and views currently have 100% coverage. Closes #50 (closed). As every bit of code needs tests a general test issue is not required. Code that wants to be merged into master should be completely tested anyway.

backend/.coveragerc

 [run]
 branch = True
-source = core,util
+source = core,util.factories
 omit =
     core/migrations/*
     core/apps.py