ENH: Easier version downloading

Ready for review/merge @rkmaddox. This does two things:

1. Hides the ugly "copying/building" outputs of `setup.py` when downloading an `expyfun` version, and adds more helpful lines about what was downloaded where.

2. Makes it so the default is to download the latest `upstream/master`. I have a feeling this is what users will want to do 90% of the time. It's what I've wanted to do every time I've used the function.