Add support for serializing/deserializing geometries via yaml

This patch add support for serializing/deserializing geometries to YAML.  This is helfpul for setting up fixture files in Rails.  Note it assumes the use of psych (instead of the deprecated syck) and ruby 1.9.  Ruby 1.9 is required due to a call to force_encoding, but that could be fixed easily (and then syck support would have to be added though).