Refresh of SQLAlchemy support

I moved forward the SQLAlchemy branch to support all the changes happened to Eve since last commit to the branch, or at least the ones affecting the find methods of the data layer.

The current state of the SQLAlchemy support is a bit of a regression since I haven't updated the previous implementations of the find_one and insert methods in the data layer to comply with Eve 0.4.

Mostly though I kicked off some tests for SQL DBs by porting them from the get method tests of Mongo.
Hopefully this means we can make progress on the branch a little faster.