CQL exec call using prepared statement

This should fix issue #28.

The exec call now uses prepared statements. This allows for proper support of numeric types. It is also the basis for future creation of a prepared statement call. 

What does not work:
* decimal types
* map types
* list/set types... might work, but untested.

@mkjellman could you do a basic once over of the code to make sure I didn't do anything crazy?