Allow viewer data to be updated in real-time

[r-b-g-b]

This commit makes a few changes that allow you to refresh the brain data using something like the following code... client = cortex.webshow(data) # data is a cortex.Volume, client is a JSMixer object # make a new volume, data1 client.addData(data1=data1) # data1 is the key, data is some new cortex.Volume