Replaced  ad-hoc implementations of kernel and string extensions with ActiveSupport's

Did not replaced Hash extensions atm. The implementations are not the same. Will look into it. 

Also, while resque/core_ext/string.rb is greatly simplified  and piggyback son ActiveSupport, I did not fully get rid of it because requiring string ActiveSupports string extensions or even string/inflections adds a dependency on the I18n gem. Did not consider worth adding that dependency. What do you think?