Add a license file

[mjsalinger]

Please add a license file to demonstrate what type of open source license this falls under. Under current law in most countries, this code is proprietary and falls under exclusive copyright because there is no license attached.

See https://help.github.com/articles/open-source-licensing#what-happens-if-i-dont-choose-a-license

[paixaop]

Thanks for the heads up. MIT License added.