Progress-tracking, jsdoc, require native Promise support, dependency updates

[davidnbooth]

For some reason the Worker kept hanging on pdf generation until I removed the "es6-promise" dependency. I'm not sure if you'll want to merge this change because it would break the library for people who DO need the promise polyfill, but I made some other changes as well that I think are useful.

There's a rough implementation of the progress tracking feature and jsdoc comments for all the functions/methods.

Even if this pull request isn't appropriate as-is due to the promise polyfill issue, I'd love to connect about how I could help with this project if possible!