Activities export

[just-some-random-guy]

Thank you for starting this! This is already great! The one feature I think would be most helpful to me is to add the ability to export recorded activities to .tcx files. This could previously be done through the fitbit dashboard, allowing for easy import into other programs.

[jlai]

Added a TCX download button (click on an activity with the compass icon, look for the button in the upper right)