Add MIT license

i'm in the process of packaging this up for the [Atom](https://atom.io) text editor... and i figured i'd stand on the shoulders of your work :smile:

Before i publish it, i figured i'd make sure things are semi-kosher by making sure that the license stuff is taken care of.

If you would rather publish powershell support for atom yourself, let me know and i'll take down what i have! i'll wait a week (so until 6/13ish) and if i don't hear from you by then, i'll publish it myself.

Thanks for putting this package out!