Add support for UltraJSON

[keimlink]

UltraJSON is an ultra fast JSON encoder and decoder written in pure C. We should add transparent support for it. This means it's only used if it's installed. Of course we need additional tox environments that run the tests with UltraJSON installed because it can't be added to the requirements.

[DebVortex]

Adding UltraJSON support may be problematic. According to this Issue of anyjson, they handle errors different than other json packaged by returning an empty object.

[keimlink]

UltraJSON is throwing exceptions like other JSON libraries do too. So there is no reason not to use it. Of course there is no support for UltraJSON in anyjson, but that's not really a problem.