sync with ruby trunk
* ext/psych/lib/psych/visitors/yaml_tree.rb: change output reference
  ids to be sequential numbers.
* ext/psych/lib/psych/visitors/yaml_tree.rb: fixing string quotation
  when dumping Ruby strings. Thanks Ingy

* test/psych/test_psych.rb: appropriate tests.

* test/psych/test_yaml.rb: ditto
* lib/psych/y.rb: Document Kernel#y [Github Fixes #127]
Missed author from 706545c
* lib/psych.rb: rdoc for Psych overview by Adam Stankiewicz
  [Github Fixes #134]
* lib/psych.rb: specify in rdoc what object is returned in parser
  By Adam Stankiewicz [Github Fixes #133]