Fix iOS error by checking whether a call back exists before trying to…

[cheryjose]

For iOS devices, jockeyjs clientside is not checking if a callback exists when it tries to call them - causing the error. When a callback does exist, once it has been called it will delete it from the object of callbacks, which will mean duplicate callback triggers with the same ID will fail.

[cheryjose]

@tcoulter could you kindly check this pull request.

Thanks Chery