Simplified quickstart instructions

If you want my real opinion, it's silly to be worried about downloading a shell script as an attack vector in this case given:

* The same people are writing the guide as release the code to Hackage
* Hackage has next to no security on it
* Nothing it stopping someone from inspecting the shell script that was downloaded

My stronger reason really is the Windows users. We'll need to provide Windows-compatible instructions anyways, and once we do, it becomes a matter of synchronizing two different instruction sets, which becomes painful. (Part of my motivation in this rewrite of the quick start guide is to keep more information centralized on stackage.org.)