Add Touch instructions to Navigation Help widget

For #1846 

![image](https://cloud.githubusercontent.com/assets/3451886/3557951/401fc268-0936-11e4-8b95-d92e23b8067d.png)

Defaults to display touch instructions, then switches to mouse if it sees a mousemove before any touch input.  On touchstart, it will switch back to touch instructions for a computer with both mouse and touch.

I also added local storage logic to `Viewer` so the default behavior is to hide the navigation help after 5 views.