Added Context model and collection service, as well as updated
permissions checking and test cases to take new representation of
context into account.
Update javascript client to deal with new style context link.
Merge branch 'develop' into feature/contexts_service
Fix context attrs changing context_id to context.id, per code review.
Merge remote-tracking branch 'origin/develop' into feature/contexts_service
Fix migration to reset all `context_id` to NULL

* required for foreign key constraint to be created
Rearrange inheritance order so eager_query works correctly

* otherwise there is a 'people' query for every object due to
  `modified_by` relationship
Avoid 'Unexpected end of input' error

* /api/log_events service returns an empty body, so 'dataType: "json"'
  didn't work
Merge remote-tracking branch 'origin/develop' into feature/contexts_service
Merge pull request #63 from reciprocity/feature/contexts_service

Added new context service. Updated test cases and javascript client to use