Incorporate gif.js library

https://github.com/jnordberg/gif.js
Allow exporting animation of history as gif

When testing locally, you'll get security issues due
to the use of web-workers by the gif.js library. Use

--allow-file-access-from-files

As a command line argument to Chrome when running
locally!
Don't call dump-image when exporting to png
Show progress indication when exporting history animation
Provide some space between menu and about info
Credit the gif.js library authors
Bump version to 0.0.3a
Merge branch 'ExportHistoryAsAnimation'

Adds functionality for exporting the
entire undo history as a gif animation.