Update configuration documentation.

Also sets the region via `~/.aws/config` instead of environment
variables in the quick start guide.

cc @jamesls, @kyleknap 
Might want to make a note pertaining to how collisions (i.e. credentials being located in more than one location) are handled. Like which credentials source trumps over other credential sources? Or state something like the credentials used are the ones discovered last in the order specified.