add property to include private activities in stats

[khassel]

With the current api strava only includes data from public activities. I was not aware of this and therefore wondering about the "wrong" strava stats displaying in my mirror.

Its strange that strava don't offer stats including private activities, I think they have this function, they are using it on their website e.g. in side-by-side comparison of athletes.

Anyway, to get this stats you have to iterate through all activities and calculate stats yourself.

This PR includes a new property showprivatestats (default = false). If set to true the stats are calculated over all activities including private activities, if false stats are used as before.