Resize Taffybar window when screen config changes.

This ensures that the Taffybar window always spans the full width of the
monitor on which it is configured to be displayed, even if the screen
configuration changes. This happens, e.g., when the screen resolution
changes or a laptop user connects an external monitor.
Merge pull request #58 from rycee/resize

Resize Taffybar window when screen config changes.