Improve kinetic effect when panning fast

This PR proposes 2 changes to improve the kinetic effect when panning fast on mobile devices.

When panning fast, `handlePointerDrag` is often called only once. The current implementation only starts logging the kinetic points after the 2nd call of `handlePointerDrag`. Which means that there might be not enough points to calculate the kinetic effect. The proposed change is to start logging the points with the very first call of `handlePointerDrag`.

The second commit changes the way how the points are selected, that are used for the calculation of the kinetic effect. The previous method enforced that the first point that was logged in the delay time, was not the very first logged point (I don't understand why..?). But if there were not many points (e.g. when panning fast), the kinetic effect was canceled. Now, the animation is also triggered when the time for whole process (pointerdown-drag-pointerup) is less than the delay time.

Demo: [simple.html](http://tsauerwein.github.io/ol3/kinetic-fast/examples/simple.html) or [mobile-full-screen.html](http://tsauerwein.github.io/ol3/kinetic-fast/examples/mobile-full-screen.html)

Closes https://github.com/openlayers/ol3/issues/531
I changed it to:

    if (this.points_.length < 6) {
      // at least 2 points are required (i.e. there must be at least 6 elements
      // in the array)
I hope this is less confusing.