Add support for Django 1.7

[apendleton]

As discussed in #311, this branch adds support for Django 1.7 in avocado. I imagine there might be some tweaking needed before it's ready to merge, but the big thing is that Django 1.7 doesn't support South. The ramifications here:

• This branch starts a clean set of new-style Django migrations from the current state of the models, which seems to be what's recommended in the 1.7 release notes, but still seems like kind of a pain. Old-style migrations are preserved for Django<=1.6.x, but the transition from one to the other might be awkward, and new migrations introduced from here on would (I think) need to be duplicated for both South and 1.7 migrations.
• A couple of backup-related tests test South-specific behavior. For the moment, these are skipped if the Django version is 1.7, but it may be that suitable equivalents should be written using 1.7 migrations.

[coveralls]

Coverage decreased (-3.11%) to 78.37% when pulling 49d19b2 on apendleton:issue-311 into 37e0f23 on chop-dbhi:master.

[coveralls]

Coverage decreased (-3.11%) to 78.37% when pulling 49d19b2 on apendleton:issue-311 into 37e0f23 on chop-dbhi:master.

[coveralls]

Coverage decreased (-2.99%) to 78.49% when pulling 457458a on apendleton:issue-311 into 37e0f23 on chop-dbhi:master.

[apendleton]

Okay, so Travis is passing now for Python 2.6/2.7 and Django 1.5/1.6. I want to add testing for Django 1.7, but Django 1.7 doesn't support Python 2.6, and my Travis-fu isn't adequate to figuring out how to add a Django version for only one Python version.

[apendleton]

Wait, never mind, figured it out.

[coveralls]

Coverage decreased (-2.95%) to 78.53% when pulling cf8a961 on apendleton:issue-311 into 37e0f23 on chop-dbhi:master.