Django 1.8 support

[simas]

• ModelForm requires exclude parameter.
• IPAddressField was added next to GenericIPAddressField since django 1.4 and removed in 1.8 (https://code.djangoproject.com/ticket/20439)
• django raises warning when max_length keyword argument is passed to IntegerField
• django.test.runner.DiscoverRunner is removed. Instead use django.test.runner.DiscoverRunner. And need to call django.setup() before starting test runner.

[motiejus]

Thank you for the update. Besides in-commit comments, here are some remarks:

Some changes are iteratively made:, e.g. changing the field and then adding a conditional to support more Django versions. Please separate these to different commits. That way it is easier to review and track history. Don't hesitate to make a bunch of commits. :-)

Whitespace changes should be separate commits. That helps bisecting in the future.

Looks like the GenericIPAddressField change is not compatible in case MySQL storage system is used. We have two ways to proceed: either bump the major version and add a backwards compatibility note to a README, or make a migration to keep it forwards and backwards compatible. As I don't use MySQL, for me both are fine. This is your choice. In the current master I added a placeholder for the compatibility note in case you want to use that option.