Log Message:
Update ruby-domain_name to 0.5.12.

* Update the eTLD database to 2013-06-06T23:00:56Z.
* Mention IDNA.
* Allow etld_data.rb to be nonexistent for regeneration.
* Add *_idn methods that do ToUnicode conversion.
* Minor code cleanup.
* Add DomainName#superdomain.
* Update the database to 2013-04-05T23:00:49Z. (just a reorder)
* Add a task import as alias for etld_data.
* Improve documentation.
* Make sure to install a required version of bundler.