make toolbar buttons appear/disappear on mouseover

This PR slightly re-does the way buttons are created.  The result is that buttons are hidden by default, and are shown when the mouse hovers over the plot.

The nice thing about this is that it gives a visual indication that there's some sort of interactivity available: I've had several people tell me that mpld3 is not working for them because they don't notice the faint icons in the bottom left of each plot.

This might break the brushing example, but I plan to eventually wrap that into the main package anyway.

Note that to test this out, you'll want to use ``python process_testplots.py --local`` with the branch.