Optionally extend ol.Coordinate to the third dimension

We continue to inherit from goog.math.Vec2 instead of goog.math.Vec3
because goog.math.Vec2 better represents a position in space, with
appropriate functionality, whereas goog.math.Vec3 represents a direction
and magnitude.
Use NaN as default value for Z
Merge pull request #401 from twpayne/coordinate-3d

Optionally extend ol.Coordinate to the third dimension