Replace the UUID interface with a string typedef.

Also renamed registeredUUID to canonicalUUID.

Fixes #38.
I'm still hopeful we won't need to expose this giant list of things in the actual API. 

AFAIK, it's not possible to express, for instance, "mass.kilogram" in WebIDL without first defining a "mass" attribute (i.e., you can't have "foo.bar" things).