Unit tests for K2 storage framework.

- Does NOT test driver implementations.
- Only verifies that the framework is working as it should.
Changed utility methods to static.
Update comments for K2StorageTest.
Merge branch 'master' of https://github.com/google/K2
Add more comments to MockStoreDriver.
Small comment edit.
Merge pull request #15 from darylseah/master

Unit tests for storage framework.