Add an option to only update hash on demand

NOTE: This commit breaks the previous init method signature.
Backbone.HashModels.init now expects and options hash as its
one and only optional argument.

Automatc hash updates on model attribute changes can now be
disabled by calling

  Backbone.HashModels.init({
    updateOnChange: false
  });

You can manually update the hash whenever you like by calling

  Backbone.HashModels.update();