Basic implementation of HTML5 backup

Hey guys,

spent some time on fixing the issues you were discussing in #23 and my solution seems to work fine.

It's pretty straight forward:
- go to first page and switch to single page
- fetch currently opened page by putting together the canvas blocks based on their size and absolute position (pretty much like tomchiverton's solution)
- if possible: go to next page and wait until it is drawn
- else: zip the whole thing (replaced some of the old zip code here)

I'd appreciate if you gave it a look. May be a little buggy, it's just a sketch.

Thanks!