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.
All of the above code sets up a walker object for `libgit2`, but it's not used since below we just shell out to git and read all of the commit strings into memory at once. I was going to say we could instead use `libgit2`'s walker to iterate over the commits one at a time but it looks like the default `Iterate` method in `git2go` *also* [reads in the commit object](https://github.com/libgit2/git2go/blob/master/walk.go#L179) which could add some unnecessary overhead.

We could just implement the [first part](https://github.com/libgit2/git2go/blob/master/walk.go#L169-L174) of that function ourselves and defer reading the commit object when we actually need it (which looks like isn't until `printResult` here).

Otherwise, we can drop the above code and the code below should continue to work fine.