Js animation

- Added modified version of JSAnimation original class by @jakevdp for more general purposes (htmlwriter.py was the only file modified). Additional functionality from original class includes: frame number box addition in animation windows, new function to generate general filenames (important for clawpack),  frame directory control, animation speed control, frame width control and option for additional html code in the preamble (Plot Index link). This modified class will be submitted for a pull request to JSAnimation.

- Plotpages.py was modified to substitute the old animation style with the new JSAnimation style. The original script of @jakevdp was modified to read images from file instead of plotting figures from matplotlib. A child class of HTMLwriter is added for functionality required specifically by Clawpack. The old animation was commented out, though it could be completely wiped out.