Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
Why not pass `arg_check` along to `set_model_x` and have a single `validate_model_component` method?

Like this:
```ruby
def set_model_name(model_name, validate = false)
  validate_model_component(model_name) if validate
  @model_name = model_name
end

def validate_model_component(component_value)
  if component_value.nil?
    raise # the rest of the code from `check_model_x`
  end
end
```