peewee 0.9.6 docs show transaction context manager, which isn't in 0.9.6
thanks for catching this, i pushed a new version 0.9.7 and a new git tag.

The [docs for 0.9.6](http://peewee.readthedocs.org/en/latest/peewee/cookbook.html?highlight=transaction#context-manager) show the context manager for transactions, but after installing 0.9.6 this wasn't supported.

Installing from `git:master` fixed it. Maybe time for 0.9.7?
