Fix case when map includes a symbol key in #emit_coder

Mimic what is done in #visit_Hash. Fixes #107.
Do not use one-letter variables
sync with ruby trunk
* ext/psych/lib/psych/visitors/to_ruby.rb: correctly register
  self-referential strings. Fixes tenderlove/psych #135

* test/psych/test_string.rb: appropriate test.
* ext/psych/lib/psych/exception.rb: there should be only one exception
  base class. Fixes tenderlove/psych #125
* ext/psych/lib/psych.rb: require the correct exception class
* ext/psych/lib/psych/syntax_error.rb: ditto
* ext/psych/lib/psych/visitors/to_ruby.rb: ditto

fixes #125
change test name to be more helpful. fixes #124
Merge pull request #122 from tjwallace/fix_coder_emit_hash

Fix case when map includes a symbol key in #emit_coder
Remove remaining .swp file
Update rbx install docs
Merge pull request #131 from sheerun/fix/naming

Do not use one-letter variables
Merge pull request #141 from kdisneur/fix/remove_swp_files

Remove remaining .swp file
merging from Ruby
merge from Ruby
* 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
updating manifest
Replace outdated ruby-head with 2.0.0 on travis
merging from ruby trunk
* ext/psych/lib/psych/visitors/yaml_tree.rb: register time objects so
  they are referenced as ids during output.
* test/psych/test_date_time.rb: corresponding test.

Fixes #146