Move InteractiveImage to Bokeh

[jbednar]

The InteractiveImage class provides very general support for making a Bokeh plot out of any callable that can return an image. datashader is one place that comes up, but there are an infinite number of other possible applications, such as raster downsampling in general. It would be good to make a pull request for Bokeh so that this functionality is available to other Bokeh users as well.

Moving it is probably premature, though, as we are just now starting to work on legend and hover tool support (#90, #91) that would necessitate changes to InteractiveImage. Specifically, we are expecting to supply not just the image, but some metadata useful for constructing keys and hover information for Bokeh to use. We might do this as an InteractiveImage subclass (to keep the basic image support cleaner), but in any case, perhaps best to wait until after we've worked out the best way to do that, since we'll probably at least want to change the expected function return type from an image into a dict or other container structure, at least optionally.

[jbednar]

No longer in the repo; use HoloViews instead!