persistent zooming for rickshaw sliders

using the [glasnt/rickshaw persisent_zoom] branch code, make the jquery range
slider store it's handle values in the window.location.hash. Pick up these
values, if available, on build.

For adding/removing metrics on the same graph, append the hash on click to the
sidebar urls. Do not append for changing graph type or duration, since
persistent zooms aren't logical there.

Allows adding/removing metrics to keep the same zoom. Allows url sharing of
pre-zoom.

Limitation: zoom is in seconds since epoch, so over time the zoom won't have
any effect. Could add functionality to check that values are in range.