Add progress bar smoothing

[ljwagerfield]

Currently for small files:

• The progress wheel start off with a delay,
• Then the wheel quickly spins an entire cycle
• Then there's another delay at the end.

This can be improved by incorporating the setup & teardown times of the uploads into the progress, as suggested here.

[ljwagerfield]

Resolved with https://github.com/upload-js/uploader/commit/b34d2540d35cdda7fcefdc6077ad246888dfb131