Add the ability to use icon fonts for nodes such as FontAwesome

- Added fontSizeMin, fontSizeMax, fontSizeMaxVisible, scaleFontWithValue, fontDrawThreshold to Nodes.
- Added fade in of labels (on nodes) near the fontDrawThreshold.
- Added nodes option to zoomExtent to zoom in on specific set of nodes.
- Added stabilizationIterationsDone event which fires at the end of the internal stabilization run. Does not imply that the network is stabilized.
- Added setFreezeSimulation method.
- Added clusterByZoom option and increaseClusterLevel and decreaseClusterLevel methods.

Still clustering bug and need more documentation on new features

Fixed issue where  multi line labels for image and circularImage nodes would overlap the node

- Fixed issue #564 by resetting state to initial when no callback is performed in the return function.

When there are many overlapping objects, pick the last one which
is drawn at the top of the others. This is the same way the
_getNodeAt() / _getEdgeAt() methods of the SelectionMixin work.

Throwing this error was introduced recently, but appears to have been unintentionally committed (since the error message isn't helpful/relevant, and because the unexpected Error will unnecessarily disrupt the display of the timeline if not caught).