Add new level 3 fields (oy).
* modify request object to allow multiple keys with the same name (required for the level 3 LINEITEM keys)
* modify tests to expect an array back
* rename a variable in gateway.rb
* split out param validation into its own method
* modify debug.rb to spit out request URL even if there's an exception
* adds code to perform Level 3 requests for Visa/MC