Can I haz attribute :symbol please?

 Add symbol as an attribute type

* Because I'd like to have it
* had to change class check to lookup at the top level because
  otherwise it'd break (probably tries to lookup the new Symbol
  class then - suspect that String is broken in the same way)
* also added .ruby-gemset to the .gitignore

edit: implementation along the lines of String etc

Cheers + thanks,
Tobi