Fixes to documentation of Arbor.js
Javascript implementation of synapse clustering, using a partition-based
tree traversal approach that cuts down computational time enormously
compared to the ~O(n^3) graph-based approach currently implemented
in the server side in python + scipy.
synapse clustering: implemented Casey's density hill map, that computes
the clusters synapses fall into when using a specific bandwidth parameter.

The UI is not yet completed.
Added functions to retrieve lists of treenode IDs representing synapse clusters.
Created Colorizer as a util. Copied from SelectionTable.
SelectionTable uses Colorizer in utils.
WebGLApplication: synapse-clustering as a synapse coloring mode.

Uses a bandwidth parameter, in nanometers, specified in the options.
Added opacity field to SkeletonModel.