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

Next steps:
* [ ] Beef up the URI parser
* [ ] Integrate signing into the URI
* [ ] Introduce a url-safe base64 #to_param