Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
Now that I think about it is `arg_check` ever nil? In the documentation it's defaulted to `false` in `#new`: http://ruby-doc.org/stdlib-2.1.1/libdoc/uri/rdoc/URI/Generic.html#method-c-new.
And if it is nil shouldn't we just treat that as a falsy value?