Added reschedule:interval method to CCNode

Added reschedule:interval method to CCNode.m. When you call the
reschedule:interval: method using a selector that is already scheduled
this method will update the interval.  It may be useful to use this
method at certain times rather than using the  schedule:interval:
method (which will unschedule and then reschedule an already scheduled
method).

If you call reschedule:interval method and pass in a selector that was
never scheduled, this method will schedule the selector for the first
time.