Changes in demo app to take advantage of GCE enhancements.

This fixes #36 

The following commits complete the first check in the checklist of the issue #36 (this is the first time I use checklists, I would check it myself but I don't know if that's correct. Perhaps you'd like to review it first and check it when you're sure this completes that task. Otherwise let me know and I'll check it)

Sorry, perhaps I should have made this change in another PR, but it was the first thing I needed to start coding the changes to the demo app. The first commit of this PR also fixes #37 
Just a nitpick... there would be less code if you set `elevation = None` before the `hasElevation` condition and always made the same append to the output rows like this:

```python
elevation = None
if hasElevation:
  elevation = point.elevation
  if elevationInFeet:
    elevation = toMeters(elevation)
outputRows.append([track.name, ts, point.longitude, point.latitude, elevation, metersPerSecond, None, 1])
```