Project Cassandra Tweaks #1

This is the first patchset in a series of pull requests to improve the driver. This first patch actually fixes the driver so that it will work against a real cluster. There are a couple of problems addressed here:

1) The existing version can't work in a real environment because a join is used (not allowed in cassandra but makes it past our mocking)
2) Fixes the way drivers are loaded dynamicallly
3) Adds a unit test for testing adding pre/post blocks. This was necessary in order to ensure code coverage. 
If _get_block_size() returns None.  will this calc raise exception?