serialization (dtas-player)
---------------------------

* objects serialize using the "to_hsh" method (like "to_hash", but omits
  default values) and then to YAML.  We avoid exposing the fact we use
  Ruby (or any programming language) in any formats.

* every serializable class defines a "load" singleton method which takes the
  output Hash of "to_hsh"

* we avoid serializing default values to make the state file shorter and
  more suitable for human viewing and editing.