add "with:" usage example to Transaction, Session.begin()

[Mike Bayer]

No description provided.

[Mike Bayer]

http://docs.sqlalchemy.org/en/rel_0_7/core/connections.html#sqlalchemy.engine.base.Transaction

http://docs.sqlalchemy.org/en/rel_0_7/core/connections.html#sqlalchemy.engine.base.Engine.begin

[Mike Bayer]

http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#autocommit-mode

http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#autocommit-mode

http://docs.sqlalchemy.org/en/rel_0_9/orm/session.html#using-savepoint