Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
I see. It's used
* by `.build` to generate the URI automatically from an Array or Hash argument.
* by `#==` to compare value equality
* by `#hash` for act as a consistent hash key
* by `#eql?` for precise value comparison
* by `#component_ary` to return a full list of URI parts
* by `#select(*components)` to select a subset of URI parts
That's quite nice.