depend on requests (experimental)

Food for thought.

I just thought I'd try to gauge others' opinions on the idea of depending on [requests](http://python-requests.org). Since we spend a lot of effort on setting up the HTTP process and handling errors, this turns out to replace quite a bit of code. Interesting!

This seems like it might be a boon for maintainability/reliability, but I'm mainly opening this PR for discussion, not on the expectation that it will be immediately merged. (For beets, we're considering moving to requests for everything else sometime soon, so the extra dependency wouldn't be a problem. But I'm there might be other projects out there for which another dependency would be a pain.)