Added compatibility for all Django versions up to 2.1

[PetrDlouhy]

This is compilation of several branches around GitHub that target compatibility with newer Django versions.

The #60 and #64 patches are part of this, so I suggest pulling them first.

I finished this by fixing tests in all Django versions (1.4-2.0).

[PetrDlouhy]

@tkaemming Could you please manage this? First pull #60 and #64, than this and than release it as version 2.2.0?

[vasiltabakov]

You can try using another package called 'django-hosts' if you can't hold your breath for django-subdomains.

[jeffreybrowning]

@tkaemming Please merge.

[toniengelhardt]

@tkaemming Yes please.

[PetrDlouhy]

@tkaemming Can you please merge?

[PetrDlouhy]

@tkaemming I added testing also for Django 2.1, which passes.