Add support for Django 1.7

[apendleton]

As discussed in chop-dbhi/avocado#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 49d19b2c53ea70ed940eb3738a1c69c40934884b on apendleton:issue-311 into 37e0f23cd35822827b38785b9981c6b18d6152de on chop-dbhi:master.

[coveralls]

Coverage decreased (-3.11%) to 78.37% when pulling 49d19b2c53ea70ed940eb3738a1c69c40934884b on apendleton:issue-311 into 37e0f23cd35822827b38785b9981c6b18d6152de on chop-dbhi:master.

[coveralls]

Coverage decreased (-2.99%) to 78.49% when pulling 457458a8789d94576d5555df169be9d28b24772f on apendleton:issue-311 into 37e0f23cd35822827b38785b9981c6b18d6152de 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 cf8a961a4fb4f1f223d5ade9f7f1151c00b6a68d on apendleton:issue-311 into 37e0f23cd35822827b38785b9981c6b18d6152de on chop-dbhi:master.