Touch events support

This PR adds support to touch events (W3C and IE).

`ol.MapBrowserEventHandler` triggers three new event:
  * `TOUCHSTART`
  * `TOUCHMOVE`
  * `TOUCHEND`

The mouse and touch events are splitted (`handleMouse*`and `handleTouch*`)

Because we use an old version of the closure lib, I had to backport the IE events names into the temporary `ol.MapBrowserEvent.IEEventType` enum.