Enhancement X-Ref-Modified

- Adds the X-Ref-Modified to block's GET end point
- X-Ref-Modified is the last modification time of the reference data for the block
  - stores the data as 'reftime' in the metadata back-end
  - reftime is the UTC Unix Epoch of modification time.
    - SQLite by default uses UTC
    - Cassandra has a Unix Timestamp function gets the time data from 'now()'; unclear if 'now()' is UTC or local to the server.
    - We explicitly get the time for MongoDB using Python's time functions to ensure it is UTC Epoch.
- Updates SQLite Test suite to use self.assert* instead of assert &lt;expression&gt;
Wouldn't it be more "correct" here to simply change the query to update the timestamp using cassandra's "now()" function? I'm curious if this would even work against a real deployment.

Plus, it looks like this wouldn't be a correct implementation since cassandra expects an integer to millisecond granularity, no? 

See here:
http://stackoverflow.com/questions/16532566/how-to-insert-a-datetime-into-a-cassandra-1-2-timestamp-column

Yeah, it's for an older version of Cassandra but I wondrer if this still applies? I would love to see this dev tested against a real cassandra instance to ensure that it's behaving appropriately.