Added db schema support
I opted for a slightly different refactoring which you can see in cfc7c3db6987cfe1ad087d645477bd1cb8c864e8 and 7a831badaa43347fa717718c9669f617bb5949e4. The end result should be the same as what you have here.

I also chose `schema` instead of `db_schema`. `db_table` is a holdover from Django, but going forward I'd like to switch it to just `table`.

Thanks for your work. Additionally, if you'd like to submit code in the future, please shoot for pep8 and 79-char lines.
 I didn't receive your comment. I will try to follow the 79-char lines rule ;) (I already tend to respect PEP8)

Added db_schema do Model meta options. I've refactored Entity to skip empty strings (e.g. when db_schema is empty or None)
