Added license file

What do you think about adding a license file? I noticed Octokit [using one](https://github.com/octokit/octokit.rb/blob/master/LICENSE.md) and reference it in their [readme](https://github.com/octokit/octokit.rb/blob/master/README.md) as well.

[Looks](https://help.github.com/articles/open-source-licensing) to be recommended by GitHub help too.