Simplify the default recipe and attributes

* Use platform_family attribute to expand platform support.
* Remove unused or deprecated attributes (see section in README).
* Use Oracle's VirtualBox package repositories for Debian / RHEL, and
the Opscode apt/yum cookbook resources accordingly.
* Add Vbox::Helpers module in libraries/.
* Add name to metadata.rb
* Add additional platforms supported to list
* Add dependencies on required per-platform cookbooks (required for
Chef 11's chef-solo).
* Update README.