to_yaml support & yaml_db clean export

[Sudrien]

Currently an .to_yaml of a activeuuid::UUID looks like

• • !binary |- AEp9di+VR7yEQ2gUPQ9pTw==

The human-readable (.to_s) version would be much better for serialization and inter-database compatibility. In my specific case, moving data from MySQL with activeuuid to Postgres with integrated UUID support with the yaml_db plugin.

In my research so far, it looks like the answer may be defining the datatype in the style of lib/yaml/rubytypes.rb - but the versions I can find look like outdated code.