Add Django email testing support

This commit adds support for inspecting emails sent that were sent by the Django server.

I would appreciate some help in getting my tests integrated into the existing test framework. Currently the tests are in `lettuce/django/tests/functional`. 

You can manually verify this code works by going into the `lettuce/django/tests/functional' directory and doing `make test`. There's a test_email.py there that is supposed to do the same thing, but nose is failing because of a Django-related issue.