Support for Ruby 2.x.x plus various other features

Adds the following new features:

* bumps Ruby and RubyGems to versions 2.0.0-p247 and 2.0.6 respectively;
* checksums the tarballs;
* fixes downloading from the `ruby/2.0` or whatever folder under `ftp.ruby-lang.org/pub/ruby` the tar-ball lives based on the major.minor numbers;
* installs to `/usr` rather than `/usr/local` by default, but you can override;
* builds Ruby with parallel compile jobs, ten by default.