Fixed NonHierarchicalDistanceBasedAlgorithm

In some cases it counted one item in several clusters
There were two problems:
1) Typo in NonHierarchicalDistanceBasedAlgorithm.m
2) GQuadItem couldn't be used as NSDictionary key (had to override `-isEqual:` and `-hash` methods)
Original discussion: https://github.com/googlemaps/google-maps-ios-utils/issues/2