"Minimap" pan/zoom widget

Added a "Minimap" pan and zoom widget that is effectively a replacement for the RangeSlider widget. The minimap has functionality similar to Google Finance's stock charts whereby you can see the entire dataset in a vertically-compressed view and can manipulate a viewing window that controls what is displayed in the main graph.

Major changes:
- Extensions.html has been modified such that the Minimap replaces the RangeSlider. I chose to do this because the Extensions example is the best way to show off the minimap (since it responds properly to changes in Renderers and such), and the RangeSlider becomes redundant if left on the page.
- README.md updated to include this new functionality.
- Makefile includes the new class, and JSHint is happy.
- Unit test added to confirm basic instantiation of the class.