Go rewrite

Wanted to learn Go, so here we are.

This is probably a bad pull request, since it removes some features and doesn't add much to performance. On the other hand, no one cares about this repo so :notes: *I can do whatever I wanttttt* :musical_note:. All the performance hits are incurred through libgit2, so the Ruby and Go versions are more or less on par at the moment, though I have thoughts on that.

The output does look a little flashier now, though. Here's what we've got on our main GitHub repository at the moment, which takes about eight seconds or so to traverse the 200k commits:

![screen shot 2014-12-13 at 12 08 00 am](https://cloud.githubusercontent.com/assets/2723/5423095/21e22738-825c-11e4-9528-622994f8c69d.png)

I still need to figure out the binary situation, but that'll be cooler than demanding people install fucking Ruby.

Fixes #1.
Ahhhh... yeah I [changed that yesterday](872df762dcd29757e415e62181e65ea225895142) but forgot to yank the walker code out. Shelling out was faster and I was wondering why that was; reading the commit object makes sense as the additional overhead.