Add storage API finalise op & start memory backend
Implement first few operations for in-memory storage
Initial document support for in-memory store

Implement and test the operations on initial documents for the
in-memory data store.
Test from last should not be "pending"
Actually implement mappend for Diff
Diff support for in-memory store

Implement and test (with truly trivial data) the operations supporting
diff values for the in-memory storage backend.
Use multiple entities to test in-memory store
Merge pull request #25 from anchor/wip-memory-storage

Implement in-memory storage backend