Add support for more datasets

[PavlosMelissinos]

CamVid, Cityscapes or SUN, which should help with #11 (cityscapes is the most recent one).

cocostuff, as a first stress test for #8

[PavlosMelissinos]

Progress report:

Still working on multiple datasets, at the moment I'm still in the process of polishing the dataset API (check out the datasets_overhaul branch for progress on that front).

Thing is, that branch and the whole dataset API thing should really not be a part of this repo, that's why I've not merged yet. However, the code works and I think some of my design ideas are decent so I've also opened an issue on keras-contrib to discuss the API. Eventually, keras does not have to be a requirement but it's a start.