add license header

It would be nice to have `Bacon.js` file with license included. MIT license requires its distribution with every copy of software [source], so this commit will satisfy license in circumstances of taking single `Bacon.js` file directly from GitHub and including it in combined/minimized file (like affter using of *r.js*, *almond* and *Uglify*).

`Bacon.min.js` could also include license (anyway, this license header comment could be easily stripped by user). I can send another PR for that if you're interested.