Add license to the code.

[tian2992]

I would like to contribute to the code of Tether, but I am doubtful of the license terms on which it is released. Could you please add a license?

Regards

[MattRix]

Hmm good question. The code at least is MIT for sure, not sure about the art and sounds since they weren't made by me, but yeah, definitely feel free to contribute to it.