Support multi-colored scatter plots.

Scatter plots should allow supplying an array of colors to choose the marker and outline color. This is different than a color map in that the color is not necessarily a dimension in the data. It could in fact be an encoding(such as a level, or a brush). Following the example of a the marker_size array, I've made a few changes to the scatterplot class to support this. I've tested this using the scatterplot demo and have observed no change in the behavior when an array is not specified for the color. A new example which generates  a plot with random marker and outline colors has been included.