ENH: upgrade url conf imports

django.conf.urls.defaults import * --> django.conf.urls import *
BUG: DVCSError Invalid signature on Windows platform

Bug: Unable to create snippet submissions (create a repo) on Windows platform
Exception raised: DVCSError;
Exception value: DVCS Command Failed 255: Invalid Signature
ENH: use django transaction 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!