Sprints: SciPy2014 sprint ideas

• Docs on notebook CSS/HTML customisation
• Docs on writing a notebook extension with a toolbar button
• Document IPython architecture overview (finish up this PR)
• Generic source code display mechanism
  • Loading from files and URLs
  • Handling different languages
  • Syntax highlighting
• Cell metadata & tagging
• Collecting user data ethically
• UI for selecting heading cell levels, other than having every level as a separate cell type
• Enhance security for widgets, by enclosing widgets in sandboxed iframes