making the sql branch depends on sqlalchemy only

Hi, we are using **eve** with sqlalchemy branch in one of our projects (starting another one soon). Our main improvements/changes in this PR are focused on removing direct dependency between the models and flask-sqlalchemy library; so using only sqlalchemy base classes.

This has at least one major advantage for us (I hope others will find it handy, too):
 * data-layer models (pure sqlalchemy) can be shared between applications; one can have a working pyramid/flask/django application and **eve** that is doing just the REST stuff

We are willing to support this branch if it's going to be accepted.