Add failed_when module variable.

Implementation note: Ternery operator trick for python prior to 2.5 is used.
(test and [when_true_value] or [when_false_value])[0]
http://stackoverflow.com/questions/394809/ternary-conditional-operator-in-python#comment1466794_394887

Hi @mpdehaan
Here is a pull request in response to:
https://github.com/ansible/ansible/pull/4072#issuecomment-24121045

Could you take a look?
Thanks!