Switch from JavaScript to CoffeeScript as package language

[Glavin001]

Use http://js2coffee.org/

Since Atom is written in CoffeeScript and most packages for Atom are also written in CoffeeScript, it is becoming increasingly advantageous to switch to CoffeeScript for the package's core language.

[Glavin001]

I was hoping that keeping the package language in JavaScript would allow for more contributors, however as of late there have not been many contributors. This may be because atom-beautify is already at a working state and most users have no further desires (their language is already supported). Also, user and developer document needs to be updated (see #58).