Add Django 1.6 atomic transaction compatibility

To fix issue #15, add atomic transaction compatible codes which introduced from
Django 1.6

Note that somehow the test code (`test_admin.py`) did not raise any
exception even there were the issue #15 thus I'm not really sure this
commit fix the issue #15.
Merge pull request #16 from lambdalisue/issue15

Add Django 1.6 atomic transaction compatibility