Add touchstart events in addition to click events.

Clicking clustered or normal markers should trigger the opening of a popup. On
touch devices a touchstart event fires but not a click event. Duplicate the
event handling for click events so that they work with touch events also.
Register multiple events to function.

Openlayers allows mapping multiple events at once. Since we were nearly
duplicating the same function 4 times, this could be simplified by registering
click and touchstart events to a markerClick function and passing in slightly
different objects.
Merge pull request #102 from lliss/topic/marker-popups-on-touch-events

Add touchstart events in addition to click events.