Fixed demo/demo.py such that it behaves like example.html

Added the drawGraph function from example.html to demo/demo.py such that
the , and . keyevents allow the user to move from one set of graphs to the
next.  Modified the function so that it does not loop.

Changed the d3 import to the minified version (as in example.html)

Without these changes, the index.html generated by demo.py only showed a few graphs and did not allow me to switch between bamcheck files.