Support for django 1.7

I opened this to start a discussion and work on the support for django 1.7

### Want to help out?
Checkout the branch run the tests, help fixing if you can. Comment in this thread to let people now what you are working on. **Send pull-requests to this branch(dj1.7)**

### Todo
- [X] Add it to the test environmnent
- [ ] Add meta info about the support
- [x] Fix app loading
- [x] Fix item `RuntimeError: Conflicting 'item' models in application 'thumbnail_tests'`
- [x] Fix backwards compatibility to 1.5

### Stuff from release notes that might be relevant
> * Storage.get_available_name() now appends an underscore plus a random 7 character alphanumeric string (e.g. "_x3a1gho"), rather than iterating through an underscore followed by a number (e.g. "_1", "_2", etc.) to prevent a denial-of-service attack. This change was also made in the 1.6.6, 1.5.9, and 1.4.14 security releases.