[WIP] PyPy compatibility: 

I need to test it a little more yet, just dropping here for early feedback...
Because pandas do not work on PyPy, I made pandas really optional (in master it is a dependency right now) and skipped some other test because NumPyPy does not pickle arrays yet.
Basically a lot of things seems to work, but I have to test it more deeply to know the extend of the functionality...