Add started_at, distance_values and position_values

[janpipek]

If you want to draw your track on a map or plot distance vs. altitude, distance_values and position_values are necessary. I am also adding started_at for completeness as for some people (me?), this might be more relevant than completed_at.

Hope that this helps to improve your lib. Please consider whether the position format as (lat, lon) tuple is suitable.

[janpipek]

Note: I am testing the library against TCX files exported from Polar Flow. Hope that brings another level of compatibility.

[vkurup]

Thank you for your contribution. I personally don't use this library much, but I see the value of the features that you've added. 💯 (I'll fix the 'lint' error that TravisCI is reporting in a separate commit)

[janpipek]

You are welcome. I don't use the library on daily basis either, but being able to read my Polar exports is always useful.

All the best,
Jan