Deceleration bug causes multiple timers to be created, run indefinitely

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Enable deceleration
2. Drag and release twice, quickly
3. Deceleration timer is started twice (once for each touchesEnded), the 
latter overwriting the reference to the former.

Fix is to wrap the timer starting code in a test:

        if ( !_decelerationTimer ) {
            _decelerationTimer = [NSTimer scheduledTimerWithTimeInterval:0.01f 
                                                                 target:self
                                                               selector:@selector(incrementDeceleration:) 
                                                               userInfo:nil 
                                                                repeats:YES];
        }

Original issue reported on code.google.com by [email protected] on 11 Oct 2010 at 5:14