igc: Added basic Writer class
igc/writer: Added write_logger_id()
igc/writer: Added write_date() method
igc/writer: Added validate parameter to write_logger_id() method

Allow for uncommon/invalid logger ids
igc/writer: Extracted write_fr_header() method
igc/writer: Added subtype_long parameter to write_header() methods
igc/writer: Added write_pilot() method
igc/writer: Added write_copilot() method
igc/writer: Added write_fix_accuracy() method
tests: Simplified format
tests: Fixed Python 3.x compatibility
igc/writer: Added write_glider_type() method
igc/writer: Added write_glider_id() method
igc/writer: Added write_gps_datum() method
igc/writer: Added parameter defaults to write_gps_datum() method
igc/writer: Added write_firmware_version() method
igc/writer: Added write_hardware_version() method
igc/writer: Added write_logger_type() method
igc/writer: Added write_gps_receiver() method
igc/writer: Added write_pressure_sensor() method