Added support for MultiGetSlice for Counter columns and super columns

Added support for MultiGetSlice for Counter columns and super columns

Quick question though: I haven't been able to get any of the unit tests that connect to a live Cassandra instance to run due to errors with the CassandraDatabaseSetup.cs bootstrapper throwing errors - I think it has to do with the version of Cassandra running on my machine (DataStax Community Edition v1.1)

Could someone try running these and see if my new unit test  `Counter_GetSlice_Columns()` passes? I've been able to verify that the behavior works as expected inside my application now that I've made this change, but I don't like the fact that I can't get 80ish of the built-in unit tests to run on my system :(