BokehJS demo docs fix-ups

Fixes #694 .
This is not actually quite what I had in mind. There are other interesting things that we should point users to besides just de demos, for instance bokeh.pydata.org and maybe the bokeh GitHub site. I think the best course of action is to serve an actual `index.html` file, that has links to the demo html files (maybe with a little description of each) as well as those other sites.