Adding PostScript to GitHub languages.

Since [pygments](https://bitbucket.org/birkenfeld/pygments-main) has been providing syntax highlighting for PostScript for [over two years](https://bitbucket.org/birkenfeld/pygments-main/commits/d4d11936bc90), it seems long overdue for GitHub. A [previous attempt](https://github.com/github/linguist/pull/61) failed, I've tried to avoid its mistakes. A few remarks:

* I insist on PostScript being a programming language: see [Wikipedia](http://en.wikipedia.org/wiki/PostScript). The sample I provided is also meant to prove this.
* "Encapsulated PS" is not a language on its own, there are just semantic differences between PS and EPS. I also left out the very rare extensions `.epsf` and `.epsi`.
* PostScript is often generated (by `dvips`, for example). Nevertheless, I didn't change `lib/linguist/generated.rb`, since I am not aware of any common practice to commit such files.