adds git.exec, its tests and docs

Fixes #37.

Apologies for the delay. Here is `git.exec` and a test using `git.exec({ args : 'log' })`. I was going to add the documentation on this PR but then it would be nice to also have docs for `git.status` and/or `git.log` so I thought to do all that on another PR. That is, still, I'm not sure if you want those here.