Added gpxString properrty to DailymileActivity
Reverted the gpxString property from DailymileActivity, in favour of a DailymileActivityRoute property, and coupling of the GPX framework to the Dailymile library. At first I was hesitant to do this, but GPX is a format that would rarely, if ever, change. Plus training activities are tightly coupled to GPX data anyway, that’s just how it is.