Make Into a Ruby Gem

It's quite simple to make rbSFML into a Ruby Gem. This has a couple advantages: the gem could be hosted on rubygems.org for easy distribution and it makes rbSFML easier to install/uninstall since you can use the gem package manager.

Ruby gems have support for building native extensions and including documentation (which we would want for rbSFML) but I haven't figured out how to do those yet.

The biggest barrier to this pull request is the 'lib' directory used for building rbSFML. The standard folder for Gem files is also 'lib', so there's a conflict. If you remove the requirement for the SFML include and lib folders to be copied over (see issue #43), then all you have to do is change SO_DIR to 'lib/sfml' and the Gem can be built trivially.