NoneError is raised prior to insert in database

Now, __storm_pre_flush__ is implemented. For now, we traverse the columns and we set whatever column that is NOT NULL (allow_none=False) and does not have a default to None. By doing this, storm raises a NoneError immediately. 

Storm's default behaviour was inserting a placeholder value like 0, 0.0 or an empty string.

This solves issue #28

PS: Docs updated.