Add License

[mattfarina]

Can you please add a license such as Apache 2, BSD, or MIT. Without a license we don't know the terms of the code for use in our own applications and libraries.

Until there is a license I can't use this package.

[amattn]

would love a license! we're having an audit and I'd prefer to not rip out all of our asserts from our code base!

would you accept a fork if I added one?

[amattn]

Never mind, just saw MIT License in big letters in the read me. was looking for a LICENSE file or similar... sorry about that.

[aarongreenlee]

I was also looking for a license file today. Glad to see this issue. Thanks.

[duanzhao-dz]

where is the url of the license