add examples of installing rubies with ruby-build

One of the things I miss from rbfu is the copy and pasteable ruby-build instructions on the main page.
fix broken directory paths

I copied and pasted those examples from the rbfu page, like an idiot.
add details for installing to /opt/rubies

Also, rubyies should be prefixed with ruby-
change ruby-build instructions to match setup.sh

use ruby-1.9.3-p392, jruby-1.7.2 and rubinius-2.0.0-rc1 to match scripts/setup.sh
Merge pull request #108 from AndrewVos/master

Add examples of using ruby-build to install some rubies