Add a license

[johnbotris]

It would be great to have this code licensed to more freely enable the use of some of this code in other projects (I'm particularly looking at the lexing/parsing code). The MIT License would be my personal suggestion

[dagit]

Thanks for pointing this out. That's a bit of an oversight on my part. I added a BSD-3 license, which I believe is compatible with the MIT license. Let me know if you have any problems with this.