GH-95: Add SQLAlchemy storage backend

[markhobson]

Addresses #95 by adding a storage backend for native SQLAlchemy rather than Flask-SQLAlchemy. Based off the existing Flask-SQLAlchemy backend implementation.

Submitted for early feedback, so no docs yet. Interested in:

• whether this backend would be considered
• a better naming strategy (should sqlalchemy be renamed to flask_sqlalchemy?)
• the general approach

Happy to refine further if welcome.

[ddorian]

I would generally prefer data to be in a json(b) column so it's easy to query and read. Think find/delete all sessions of a user.