DownloadProvider: Add transfer speeds to notification

When downloads are in expanded mode, the bottom text will include the current transfer speeds.
Screenshot: http://i.imgur.com/xqLRvMo.png
DownloadProvider: Add transfer speeds in notification

This will show transfer speed in notification when in expanded mode.
For multiple files, the combined speed are displayed.

Screenshot: http://i.imgur.com/xqLRvMo.png
DownloadProvider: Add transfer speeds in notification

This will show transfer speed in notification when in expanded mode.
For multiple files, the combined speed are displayed.

Patch set 2: Cleaned up the code
Patch set 3: Fixed some errors (oups)

Screenshot: http://i.imgur.com/xqLRvMo.png