Add custom meta data parameter

[svenstaro]

Fixes #60, #61.

[davidism]

It's clear we need to be able to override everything that is passed to declarative_base. A more thorough way to do this would to be allowed to pass in a base_kwargs dictionary, which will keyword unpacked.

[svenstaro]

Any more discussion on this? Is that the last word on it?

[immunda]

@davidism is right, but let's get this in for 2.1. A more general solution can be developed 3.0 which will accept metadata as part of kwargs.