Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
This assumes exactly N args are passed. Would match the style of the other URI classes that take a single arg, either Hash or Array, and use a utility method to normalize the arg. For example:
* https://github.com/ruby/ruby/blob/trunk/lib/uri/ldap.rb#L73-L74
* https://github.com/ruby/ruby/blob/trunk/lib/uri/mailto.rb#L85-L86