Create abstract interface for communication with Firebird from driver [PYFB29] #51

Submitted by: @pcisar

FDB should be refactored to use abstract, higher level pythonic interface for communication with Firebird, and currect direct communication via Firebird Client Library should be reworked as default implementation of this interface.

This step will allow other implementations, notably direct communication using FB wire protocol or communication using new Firebird "object" API introduced in Firebird 3.0.