Add MIT License

Hey @dustin!

We're looking at using your CoAP package in an upcoming project my student group is working on. I am assuming that since you've open-sourced your code, you're willing to let other people use it! :smile:

We can't technically integrate your code into our code base without a license, since you will always have the copyright to what is here. I've committed an MIT license that would permit us to use your code without you having to worry about being held liable for.... anything.

Feel free to choose a different license or ask me questions. Thanks!