Custom PyYAML constructors

[wcooley]

It would be nice to be able to register PyYAML constructors, so that shorter names can be used, such as !gryaml.node instead of !!python/object/apply:gryaml.node.

http://pyyaml.org/wiki/PyYAMLDocumentation#Constructorsrepresentersresolvers

[wcooley]

I'm splitting this into separate issues: one for the constructors and one for the representers.