Initial work on adapters.

My goal here is for CassandraObject to become AxleObject. AxleObject will
contain adapters for cassandra, hstore, dynamo, etc. The commonality is
that each storage table has an id column and lots of attributes. In
simplest terms, each adapter implements select, write and delete.

@dimroc
Adapters almost working
Merge branch 'master' into adapters

Conflicts:
	lib/cassandra_object/persistence.rb
Fix a lot of tests
All tests pass!
cleanups
Move cql stuff to adapter
Adapter implements scope to query
Remove duplicate consistency_string
Use adapter.primary_key_column
Merge pull request #53 from data-axle/adapters

Adapters