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.


Create a new Django 1.7 project. Put these in a model:

```Python
    class Enhance_Plain(models.Model):
        field_p = models.CharField(max_length=10)
    class Enhance_Base(ShowFieldTypeAndContent, PolymorphicModel):
        field_b = models.CharField(max_length=10)
    class Enhance_Inherit(Enhance_Base, Enhance_Plain):
        field_i = models.CharField(max_length=10)

    class DiamondBase(models.Model):
        field_b = models.CharField(max_length=10)
    class DiamondX(DiamondBase):
        x_id = models.AutoField(primary_key=True)
        field_x = models.CharField(max_length=10)
    class DiamondY(DiamondBase):
        y_id = models.AutoField(primary_key=True)
        field_y = models.CharField(max_length=10)
    class DiamondXY(DiamondX, DiamondY):
        xy_id = models.AutoField(primary_key=True)
```

Then try to validate, migrate, or run the app. The new checks system will find it and deny Django from starting.