Add support for nose (standalone and for Django)

[roadsideseb]

I thought it makes sense to open an issue for this so that we can keep track of progress and discussion points.

I finally managed to get a POC discoverer for nose up and running which means it is not far of. It looks like there'll be enough overlap between the generic and Django-specific nose runner to work on both simultaneously. One slight annoyance, however, will be that for Django + nose, we'll need to have a separate Discoverer that is used in place of the existing one. There's two ways of handling that:

1. Check in the discover_commandline of DjangoProject for the used test runner in settings and switch between the default or nose one
2. Have a separate project in cricket.django.nose that overrides DjangoProject where appropriate.

There might also be alternatives to those two that I don't see. I personally think that 2. is the more appealing solution. What are your thought?

I'll update here and attach a PR as soon as I get a first version hammered out.