Need to add effects on hover

[anvk]

For cluster layer it would be great to make circle features bigger upon mouse hover as well as showing boundaries of the cluster.

[anvk]

Opened a conversation in StackOverflow about boundaries:
http://stackoverflow.com/questions/37897877/openlayers3-how-to-display-boundaries-for-ol-source-cluster?noredirect=1#comment63301009_37897877