* ext/psych/lib/psych.rb: Adding Psych.safe_load for loading a user
  defined, restricted subset of Ruby object types.
* ext/psych/lib/psych/class_loader.rb: A class loader for
  encapsulating the logic for which objects are allowed to be
  deserialized.
* ext/psych/lib/psych/deprecated.rb: Changes to use the class loader
* ext/psych/lib/psych/exception.rb: ditto
* ext/psych/lib/psych/json/stream.rb: ditto
* ext/psych/lib/psych/nodes/node.rb: ditto
* ext/psych/lib/psych/scalar_scanner.rb: ditto
* ext/psych/lib/psych/stream.rb: ditto
* ext/psych/lib/psych/streaming.rb: ditto
* ext/psych/lib/psych/visitors/json_tree.rb: ditto
* ext/psych/lib/psych/visitors/to_ruby.rb: ditto
* ext/psych/lib/psych/visitors/yaml_tree.rb: ditto
* ext/psych/psych_to_ruby.c: ditto
* test/psych/helper.rb: ditto
* test/psych/test_safe_load.rb: tests for restricted subset.
* test/psych/test_scalar_scanner.rb: ditto
* test/psych/visitors/test_to_ruby.rb: ditto
* test/psych/visitors/test_yaml_tree.rb: ditto