Enable slave binds using master db for Postgres
Use postgres slave node for queries
Add sessionmaker to use transactions.
Create Session cls to bind to slave or master nodes.

Additionally, add get_session(db, bind) method to return a session object.
Use sessions instead of engines to run queries.
Fix for using the new execute format.
Fix to Flask-SQLAlchemy
Use new session methodology.
Use new session methodology
Fix app_by_short_name to check for None obj.
Use slave_session for get queries.
Merge branch 'master' into issue-857-sql-load-balancing

Conflicts:
	pybossa/cache/users.py
Revert changes.
Use slave session for some select queries.
Use try-except-finally for slave sesssion.
Fix broken query
Use slave session
Use slave session
Use slave mode
Use slave session