Add extras require for a sqlalchemy install

[cancan101]

See http://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

Sqlalchemy is an optional requirement for accessing certain databases.

[cancan101]

Perhaps call the extra postgresql and have it require sqlalchemy[postgresql].

[Qwlouse]

I like the idea. This could be done for several of the optional requirements. I'm on vacation until the beginning of July, but I'll take a look at it then. If you prepare a PR I'll happily merge it once I'm back.