Setup script

@chriddyp, @etpinard, what do you think about throwing warnings here like this?

basically, if you (1) don't have a core package installed (2) if the core package is older than ours and (3) if you have an outdated optional package (think matplotlib)
this is what you currently get when you run `pip install requests`. i basically just ran `pip freeze > requirements.txt` to get these version numbers we're currently working with.

I've only added requests and matplotlib here as they're the most important, yeah?