Migration to django-geojson

I must admit: your serializer code was much better than the one I had in django-geojson...

So I threw out everything, and started again from yours :)
I added a couple of options I had in the previous version. Plus, the ability to serialize dynamic attributes (@property), either geometries or properties.

I did not change any default value nor anything that could break your code. So I thought you might interested in using it !

Some unit tests and coverage were setup,  so that we can both have a robust version.

Just tell me if there's anything that bothers you in the way I did it :)