This whole pull request is essentially a revert of 577e2386 and #11

Following http://guides.rubygems.org/patterns/

A previous update forcing only Nokogiri 1.6 breaks several
applications.  Since Nokogiri 1.5 still works just fine I
modified the dependency to be >= 1.6 which accounts for
Nokogiri 1.6 as well.

Added a URL parameter that is automatically set to the referrer so that the user will be automatically redirected to the page that triggered the need for authentication.

Laid down the foundation by using `omniauth-facebook` as a guide to the structure of this gem and the old OmniAuth determining what needs to be tested.