Creates a standard interface for whitelisting classes,
independent of the YAML_ENGINE selected.

This is necessary for a library to be able to register its safe types
without being opinionated about which YAML_ENGINE is selected.
fixed typo
fixed anonymous class detection in SafeYAML::whitelist! for Syck in 1.8.7
discovered freeze and dup weren't enough, so added deep versions
reorganized specs a bit
added test that round trip on whitelisted class actually works
added a couple of (currently pending) specs demonstrating that we're not quite there yet w/ SafeYAML::whitelist!