* lib/psych/visitors/yaml_tree.rb: For now the emitter uses global_tags.
  Eventually passing in a Schema should be an option.
* lib/psych.rb: Let resolve_class helper be a Psych class method.
  It is a generic function,needed in multiple places.
* lib/psych/visitors/to_ruby.rb: Refactor ToRuby to make more readable,
  and use separate logic for loading ruby tags.
* lib/psych/schema/ruby_schema.rb: Add missing ruby tags, some of
  which should probably be deprecated.
* lib/psych/schema.rb: Local resolve_class method can class
  Psych.resolve_class class method.
* lib/psych/core_ext/module.rb: Add yaml_new method to Module class.
* lib/psych/core_ext/object.rb: Object.yaml_new allocates and
  assigns instantiates variables. Also add yaml_tag accessor,
  which is necessary because different tags can map to the
  the same class.