xcsoar: Added basic Writer class
xcsoar: Added AltitudeReference constants class
xcsoar: Extracted Writer.convert_bool() method
xcsoar: Convert start_requires_arm parameter too
xcsoar: Extracted Writer.convert_timedelta() method
xcsoar: Added second test task
tests: Removed obsolete .gitkeep file
xcsoar: Added third sample task
xcsoar: Added remaining constants
xcsoar: Convert 'score_exit' parameter
xcsoar: Added assertions on required parameters
xcsoar: Fixed flake8 issue
xcsoar: Added API documentation
xcsoar: Fixed Python 3 compatibility
README: Mention new XCSoar task file writer
Merge pull request #8 from Turbo87/xcsoar

XCSoar task file writer implementation