Add openambit to TCX conversion scripts and icon

[cfabio]

openambit2tcx-nolap.py is a first test version of the script, later on it will be removed but for now we should keep it.
openambit2tcx-lap.py is working good enough, current issues are:

1. indentation of output TCX file is messed up;
2. does not recognize paused laps;
3. cardio data are missing, I plan to add them later on.

icon.png is a file we can use as program icon.

[mbernasocchi]

@cfabio what is the status on this? üshould I merge it?

[cfabio]

I still have to fix the aforementioned issues the openambit2tcx-lap.py had.