ENH: add transactions middleware

1. We need a db transaction middleware to support snippet submissions!
without this, data is saved into db even when errors are raised during
submission
2. Please refer https://github.com/scipy/SciPyCentral/issues/132
3. This middleware mostly important for submission app

Note: This middleware is deprecated in Django 1.6 but considering that we
are still upgrading to 1.5, its a long way ahead!