Display version information (Fixes issue #34)

I added to scraper.py

VERSION = pkg_resources.require("mozdownload")[0].version

in accordance with this article on stackoverflow

http://stackoverflow.com/questions/2058802/how-can-i-get-the-version-defined-in-setup-py-setuptools-in-my-package

because I was unsure what the accepted way of calling the version is. I couldn't find anything wrong with it. Hopefully, it's ok.

Thanks.