Add `base_dir` attribute

Instead of using `node['ruby_build']['default_ruby_base_path']` add own
attribute to specify the base directory for Ruby versions. Default still
to "/usr/local/ruby".
Add recipes and attributes to the readme
Support Enterprise Linux based distributions

Tested on CentOS, but should work on others too.
Merge pull request #4 from tmatilai/centos

Add support for CentOS and other EL based distros
Release v0.3.0