Fix the getCenter function

The original _getCenter()_ function does not give correct results when the points are near the 180th meridian. For example, the center of Sydney and San-Francisco should be on the Pacific, not in Africa.

I used this algorithm : http://stackoverflow.com/questions/6671183/calculate-the-center-point-of-multiple-latitude-longitude-coordinate-pairs.

The new function returns only _latitude_ and _longitude_, not the _distance_ of the diagonal.
I added the unit tests.