Feature/ar recipies

Makes abstract rendering more accessible for the common cases of making a heatmap or doing HDAlpha by providing a convenience fucntion for each.

Addresses issues:
* 1114 -- Balanced zoom is only enforced on the first rendering, thereafter the X/Y zoom ration is computed based on plot state
* 1115 -- Adds 'render_state' parameter to the server data source request.  If the render state has not changed, server downsample returns a sentinal value.  The default render_state is None, and other server downsample code works without change.