Represent Global IDs as URIs:  globalid://basecamp/Todo/1234

* Share Global IDs among collaborating apps and with non-Ruby/Rails code
* Well-known format with parsers & generators already implemented
* Pave the way to put verifier signature in the URI params, or e.g. a "?version=456" optimistic lock
Should we add tests to make sure we are assigning the right URL parts to these instance variables?