Adds coverage over drivers and db custom types

Adds more coverage tests for some portions of the driver logic and for
the db custom types. I decided to abstain from implementing some of the
optimized driver coverage tests at this time because the only thing one
can test there is that python and sqlalchemy work correctly. Otherwise,
they're just convenience mthods.
Merge pull request #147 from Cerberus98/driver_coverage

Adds coverage over drivers and db custom types