Add python3 support to YouCompleteMe

I'm making this pull request more as a request / proof of concept that this works. If this wasn't the right thing to do, sorry in advance.

Anyhow, I mainly do my python dev with python3. Since this plugin runs as python2, the jedi competer is not particularily useful to me unless it runs as python3.

So I ported YouCompleteMe to run with python3. This, atm, makes it impossible to run it with python2, but it could work in theory. Everything tested works, and jedi completes as python3.

I'm not sure how to improve this so it can exist side-by-side with the existing python2 support, but I thought it might be worth sharing. Any pointers to improve this would be appreciated if you're interested in this at all.