Ref-Time Support

- Initial support in metadata for the X-Ref-Time field by storing the current UNIX Epoch when the ref data changes
Bug Fixes

- Fixed MongoDB driver so that it passes unchanged
- Fixed Cassandra Mocking so that it passes unchanged
Bug Fix

- Cassandra needs to initialize the reftime by inserting it without changing the refcounts
Enhancement - Test Cleanup

- Update sqlite driver tests to use self.assert*
Pep8 Compliance
Bug Fixes

- Forgot the NotImplementedError for the abstract version
- Added test when the block doesn't exist to exercise the exception handling
X-Ref-Modified Header

- Update Block Model to easily get its last reference modified time
- Update Block Controller to return the last reference modified as X-Ref-Modified when retrieving a single block
Cassandra Fix

- Missed updating the Cassandra Scheme file
- Per Jamie, Cassandra won't allow reftime in the same table as the ref counter so we follow the model in Sqlite and put it in the blocks table
Merge branch 'master' into enhancement_ref_modified

Conflicts:
	deuce/tests/test_sqlite_storage_driver.py
Merge branch 'master' into enhancement_ref_modified
Sqlite Schema Version

- Schema is still version 1
- Tried to add the change as a new schema addition but setting a default value of "NOT NULL" or using strftime/strptime causes problems in the alter table functionality.
Merge branch 'master' into enhancement_ref_modified

Conflicts:
	deuce/model/block.py
	deuce/transport/wsgi/v1_0/blocks.py
Merge pull request #132 from BenjamenMeyer/enhancement_ref_modified

Enhancement X-Ref-Modified