django 1.7 support and tox setup

django 1.7 [removed contrib
middleware](https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-MIDDLEWARE_CLASSES)
so the tests need this set by default. also added tox setup to test on
django1.7rc3
django 1.6 force send default

django 1.6 no longer defaults to False for BooleanFields which causes
startup warnings.
https://docs.djangoproject.com/en/dev/releases/1.6/#booleanfield-no-longer-defaults-to-false.
this commit forces False on the send field.
Merge pull request #37 from sherzberg/1.7-support

django 1.7 support; tox setup