Adding PostScript to GitHub languages.

Since [pygments](https://bitbucket.org/birkenfeld/pygments-main) has been providing syntax highlighting for PostScript for [over three 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:

* PostScript is a programming language in the sense of being Turing-complete: see [Wikipedia](http://en.wikipedia.org/wiki/PostScript). But since it's more often used as a page description language, `type: markup` seems to fit better.
* "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). I hope that the code in `lib/linguist/generated.rb` covers all such cases.

[For a discussion, see #362. There I amended a commit which doesn't work with pull requests.]