Make legend and hover information available outside dashboard.py

The new legend support in dashboard.py is great, and once version 0.2 has been released, we should quickly move to putting some of the new code into the datashader library itself, both to reduce the size of dashboard.py (which has now grown much larger than it was initially) and to make that functionality available elsewhere (such as InteractiveImage).

Tasks:
- [x] Move downsampling function into datashader
- [x] Adapt the legend support into a general-purpose function or class for building legends
- [x] Adapt the hover support into a general-purpose function or class for showing hover information
- [ ] Add hover and legend support to (a subclass of) InteractiveImage
- [x] Add legend and hover info to example notebooks
- [ ] Use the legend support to show a color bar underneath the histograms in nyc_taxi.ipynb, to show how the histogram values map to color values.

We should also probably move InteractiveImage into Bokeh itself, if possible (issue #97), once we've made a legend- and hover-capable version, at which point we can know what the interface should be.