Authz - fix execute method and cleanup of validate method

this fixes the bug i introduced with the execute method.

Also cleans up some of the //TODO's in the validate method, 


also 
https://github.com/lholmquist/aerogear-js/compare/lfryc:AGJS-239-promosefied-authz...lholmquist:fix_oauth_bug?expand=1#diff-b20b6b27387e9ab1003bc74e39b93634R203

was slighty wrong,  at least according to how it comes back when interacting with the google api's
I'm not sure I understand this bit? Why we need to set up `xhr.response` instead of using JSON.stringify?