minor: making the switch to rubocop

Rubocop has support for JRuby and REE, pulls from a better community driven style-guide and its more easily extendable than Tailor. Rubocop doesn't run in Ruby 1.8, but I think we can live with that. Overall, its still an all around win to make the switch.

This commit removes Tailor and adds Rubocop in its place. I've also updated our guard file to add Rubocop support when it's available.

I've also added the ability to skip the code quality tests when needed by using:

  rspec --tag ~quality:true

I've tested that these changes behave as advertised in Ruby MRI 1.8/1.9/2.0 and JRuby.