Use zoom level to determine if the new region has zoomed or not

[kusamakura]

As discussed in #119.
I saw that self.zoomLevel is implemented via CCHMapClusterControllerZoomLevelForRegion, so I used that.
Please advise as to what I need to change or improve upon. Thank you! :)

[choefele]

There are two changes in this PR:

1. The algorithm to figure out if a zoom has occurred now uses the zoom level
2. updateAnnotationsWithCompletionHandler is only called if the zoom has changed

1. The code looks easier to read, but does it improve the functionality? I'd like to understand if this issue has been fixed (can you use setCenterCoordinate:animated: without changing the clustering) and wether the new code fixes Annotations are not deselected when heading changes #66 (can you change the camera altitude without changing the clustering)

2. This breaks clustering when panning the map. For example: use the example app and zoom in quite a bit (~zoom level 14) and then pan the map. This results in clusters that won't show outside the initial clustering region