Issue #19: Add heart rate to kml export.

Also fixed the import to support importing a DB exported before DB version 5.
Also modified the cadence kml export to not export consecutive points
if they have the same cadence value (same behavior as the heart rate kml export).
Merge pull request #20 from caarmen/issue-19-kml-heart-rate

Issue #19: Add heart rate to kml export.