Django 1.7 support

* ./runtests.py will now run when Django 1.7 is installed. (I tested back to Django 1.4)
* Cleared up the Django 1.8 warnings.
* Tests will run in Django 1.7. Classes that threw known Django issues, now pass, and print a warning.


OK, but I see 2 changes here:
1. `DiamondX` & `DiamondY` have an explicit PK with different names: `x_id` & `y_id`
2. if Django >= 1.7, `DiamondXY` does not use multi-table inheritance, and therefore it betrays the purpose of this test: the diamond inheritance problem

From what I can read in the Django doc you're mentioning, 1. should be enough.  No need to distinguish 1.7 from previous releases.