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. 

Have now added a second commit to this pull requests, which adds simplistic buffering of bamcheck data rather than loading all files before displaying them.  With this change it is now possible to run the demo on a directory with thousands of bamcheck files with no issues with long initial page load or with browser out-of-memory issues. 

Have now added a third commit to this pull request, which fixed the x label for the IS section from "Line Number" to "Insert Size"