Added support for Power Meter records

I have added support to retrieve Watts data from Fit files. I have tested it with a bunch of real files recorded with a Garmin Edge 500.

What I tested succesfully using Golden Cheetah 3:
Trackpoints watts
Laps average watts
Track average watts

What was NOT COMPLETELY successful:
Differences of 0-1 watts in xPower
Differences of 0.01-0.02 in relative intensity
Differences of 3-20 kilojoules (depending on the length of the track)

Everything else seemed to work fine.

Please, let me know if you try it and be nice with me, it's my first pull request on any project :)

Best regards from Spain