Correct ol externs definitions.
Add ol.source.Cluster.
Add tests for ol.source.Cluster.
Add example for clustering.
Add missing olx.source.ClusterOptions#smallestCluster property
Add missing olx.source.ClusterOptions#distance property
Remove ol.source.Cluster#getResolution function
Remove ol.source.Cluster#setResolution function
Remove unnecessary quotes
Remove property privacy in ol.source.Cluster