Make navigating the graph less terrible

Version 0.2 introduces the ability to navigate the graph view. However, the current implementation is unwieldy, to say the least.

Currently, a pair of widgets like this [<][18/12/2012][>] allow you to define the start and end date displayed. While this allows zooming and panning to be done, there is surely a more intuitive method which could be used. Ideally, this would consist of dragging the graph and using the mousewheel to zoom (at alternatively, zoom buttons overlayed on the graph)

The reason for the current design is due to the apparent lack of GTK3 support in the matplotlib version in Ubuntu 12.10, which means the graph is a generated SVG rather than a GTK3 widget.