Generate RFC4122 UUIDs

It's the standard PostgreSQL follows.

This allows client and server side id generation, with trivial syncing. This
format is also used in Backbone.dualStorage:
https://github.com/nilbus/Backbone.dualStorage/blob/master/backbone.dualstorage.js#L82-L96

Sending ids to a Postgres API raised before this commit:

    ERROR:  invalid input syntax for uuid: "ducg0"

Related links:

* http://www.ietf.org/rfc/rfc4122.txt
* http://www.postgresql.org/docs/8.3/static/datatype-uuid.html
* http://stackoverflow.com/a/105074/356060