Added GpxSection

Instead of just displaying the elevation of each point while 'replaying' a gpx route, I want to display the gradient between the current point and the next.

The GpxSection is intended to enable this, given two GpxPoints...

- the distance between them is calculated using the haversine formula (http://en.wikipedia.org/wiki/Haversine_formula)
- the gradient between them is calculated as 'rise/run' in percent.