zoomStart event and close-to-synced zoom for EventPane layers

With a zoomStart event we can notify EventPane layers like Google of
a starting zoom. This gives the Google layer a chance to start its built-in
zoom animation at the same time that the OpenLayers map starts its
animiation, resulting in a better sync between Google and OpenLayers zoom
animation.
If possible, use precalculated center for final step

This avoids coordinate <-> pixel conversion issue, and prevents Google and
other layers from drifting apart.
Fixing typo

Thanks @bartvde.
Merge pull request #1074 from ahocevar/google-zoom

zoomStart event and close-to-synced zoom for EventPane layers