Added a note about ActiveSupport to the #html_safe method.
Cleanup Rakefile doc tasks

* Remove `require ‘yard’`. The Gemfile requires yard.
* Remove unnecessary begin/rescue block around doc tasks.
* Remove Rakefile#silence_warnings.
Merge pull request #817 from haml/rake_doc

Cleanup Rakefile doc tasks
Merge pull request #786 from etagwerker/improve_method_doc_util_html_safe

Added a note about ActiveSupport to the #html_safe method.