modified 'nbtoc.py' to auto download the 'js' and 'html' dependencies when running %install_ext
Added scrolling posiibilities to 'nbtoc'. THX to @nonamenix
Added Table of Contents description/instructions
Merge pull request #8 from magican/master

Table of Contents modifications

1. Added Table of Contents instructions to README
2. Added autoresize/scroll capabilities
3. Auto download `nbtoc.js` and  `nbtoc.html` when running `%install_ext`