Parse active duration and distance

Throwing this up into a PR for testing.

I'm not sure if the active_duration or end_time attributes are getting the correct values. One of them is probably wrong so it would be nice if @plasticine could test this out locally with a TCX file that you know the real values of and let me know how you go.

I'm going to do the same by downloading some files from garmin connect to play with later.