Set abs_zero for adv_diff_test
closes #3730
Merge pull request #3733 from permcody/adv_diff

Set abs_zero for adv_diff_reaction_test
Add Executioner::terminate() (#3735)
Add Terminator UserObject (#3735)
Deduce needed postprocessors (#3735)
move logic from Executioner into Problem (#3735)
Move Terminator UO into core, new test (#3735)