Server-side colors and ISO Contours

* Support for image_rgb output from AR (supports for sever-side coloring, including HD alpha)
* Support for poly_line output from AR (supports ISO Contours)
* Tests (in the spirt of PR905)
FYI, we have is_py3 in bokeh.utils: https://github.com/ContinuumIO/bokeh/blob/master/bokeh/utils.py#L41