Remove BandwidthCalc and NativeDownloader output
Add format_bytes
Add DownloaderProgress

The class is used to report the download progress in the native
downloader.
PEP8: E501 line too long
Merge pull request #155 from jonasdt/progress

Cleanup our downloader and add downloading progress. This closes #138.
coursera: Use exception variable to log error.
README: Use regular Markdown syntax.
README: Reword instructions and mark which ones are required.
README: Make recommendation of pip stronger.
README: Put required modules before recommended ones.