Factor common base class out of in-memory collections.
Add coveragerc and rename test exception.
Remove owner_id from InMemoryCollection.
Add mechanism for getting at InMemoryCollection internals in tests.
Use InMemoryStore for api_handler tests.
Simplify InMemoryCollection a bit.
Rename InMemoryCollection _backend to _data because it is not an actual backend.
Merge branch 'feature/issue-5-refactor-in-memory-collection-backend' into develop