Add syntax checker for YAML using MRI Ruby
Improve yaml-ruby docstring

Users don't really need to know the name of the underlying YAML
implementation, as long as it reports errors.

Also link to the YAML module directly, instead of the plain Ruby front
page.
Simplify yaml-ruby test case

Merge all test cases into a single one, and get rid of all the extra
predicates.
Update credits and changelog

Thanks, @yasuyk, for this great contribution
[Fix #236] Merge branch 'yasuyk-yaml-ruby'

* yasuyk-yaml-ruby:
  Update credits and changelog
  Simplify yaml-ruby test case
  Improve yaml-ruby docstring
  Add syntax checker for YAML using MRI Ruby