Added tryToGet method to EventSetup and Record

Added the method tryToGet to EventSetup and EventSetupRecord to allow attempting to get a Record without having an exception happen if it fails. Instead of an exception, a null pointer is returned.
Merge pull request #4042 from Dr15Jones/addTryToGetToEventSetup

Added tryToGet method to EventSetup and EventSetupRecord.
Merge remote branch 'origin/CMSSW_7_2_X' into CMSSW_7_0_X