Conformant header guards

Fixes #606
Removed Unnecessary NULL-Pointer Check

Fixes #607
Merge pull request #609 from CodaFi/cpp-conformance

C++ Safety and Conformance
Fixed #586: custom headers API available in ObjC
Fixed #608: cancelled scheduled operations that are not run will not run in the future.
Fixed #605
Fixed crash when download of message failed (fixed #612)
Fixed #586 again: removed new methods and renamed existing Obj-C methods
Fixed delay type
Fixed #615: fixed issue with cancelling "perform after delay"
value back to 30 sec
Merge remote-tracking branch 'upstream/master'