Map browser event handler refactoring

This commit refactors the code of the map browser event handler. The changes are the following:

Previously the `handleTouch*_` listener functions handled both `touch*` and `mspointer*` events. With this commit we use per-event type listeners. This duplicates some code (could be avoidable), but makes it easier to introduce mouse/touch/pointer specific treatments in the future.

We now listen to pointermove, pointerup, touchmove and touchend only when needed. In this way we're assured that a `down` event was received (and that `this.down_` is set) when the `up` or `move` listeners are called. This fixes the bug @oterral reported on the mailing list: https://groups.google.com/d/msg/ol3-dev/jR844F7Nlg8/eN9dt4uATK0J.

To avoid problems in browsers that trigger different types of events (e.g. both mouse and pointer events) the `down` listeners unregister the other `down` listeners. For example, the `pointerdown` listener unregisters the `mousedown` and `touchstart` listeners, for ever.