Progress bar using Mechanize-Progressbar gem

Adds a progress bar to the downloads!
![screenshot 2014-12-20 20 06 16](https://cloud.githubusercontent.com/assets/40419/5517058/06fd6f3a-8884-11e4-8524-2a88e2e9a184.png)

One problem with this commit is its using my github version of Mechanize-Progressbar. I have a pull request to update the gem but I'm not sure if the author is really supporting it anymore. I've created a pull request for that here: https://github.com/kitamomonga/Mechanize-ProgressBar/pull/4