Develop example of multi-thumb slider design pattern

[mcking65]

The multi-thumb slider design pattern is at:
http://w3c.github.io/aria-practices/#slidertwothumb

[jongund]

I can create an example for multi-thumb slider

[jnurthen]

This is ready for review...
https://rawgit.com/jongund/aria-practices/slider/examples/slider/multithumb-slider.html

[mcking65]

With commit 84d3bd8, this example page is now complete and ready for task force review.