Fix vbarray extern

dimension argument is optional.
See http://msdn.microsoft.com/en-us/library/3kk8f869(v=vs.94).aspx
and http://msdn.microsoft.com/en-us/library/0kbz883e(v=vs.94).aspx
Merge pull request #1872 from fredj/vbarray

Fix vbarray extern
Move function declaration out of the constructor
Store touch coordinates as ol.Pixel instead of goog.math.Coordinate
Cache the result of goog.object.getCount