Add MIT license

[12joan]

Hi! I'm interested in using SocketSource in my own projects, but I can't do that without a suitable license.

This PR adds an MIT license to the ember-orbit-with-socket-source branch, in line with the package.json files for the client and server. (According to this Stack Exchange answer, the package.json line isn't sufficent to grant a license by itself.)

If you're happy for your code to be used, please go ahead and merge this PR, or close it and license the ember-orbit-with-socket-source branch as you see fit. 🙂