Add a percentage completion counter (Fixes #48)

This Pull request addresses Issue #48:

@whimboo : I took your link of mozgression/utils.py and pretty much transferred the necessary code over to mozdownload. Works well out of the box with the things I tried so far.

Here the link to the reference I used:

https://github.com/mozilla/mozregression/commit/e37a10deea53ee9ca92c0234b339ad3f7cc2e8d2

Hope it is ok.