Partial DynamicClusters implementation.

AddFeature and RemoveFeature work, but the recluster visible on pan/zoom, not cluster cached on pan, and ModifyFeature not yet finished.
DynamicClusters fully implemented, pending clean-up.

Finished implementing the DymanicClusters. Added a 'beforefeaturedrawn' event to OpenLayers.Layer.Vector so that the DynamicCluster can catch any redrawing of the features - which happen whenever a feature is rotated or moved.