The code in epub-progress.js was causing the epub file to be loaded
twice over the network, because it created a second instance of the
epub.js library. I moved all the progress percentage display code into
epub.js. Only the locationchange polyfill code was left in the separate
file, renamed to locationchange-polyfill.js.
I rewrote the percentage progress code to use epub.js events,
giving more succinct and readable code. Also, I added localStorage
caching of the epub location calculations, required for displaying
percentage progress, which can take up to 60-90 seconds to calculate on
longer ebooks. This localStorage caching approach is recommended by
epub.js in the `locations.html` example in their repo.
Signed-off-by: Ross Williams <ross@ross-williams.net>
