ME: added a duration tag in common_tags.py
ME: added duration property in 'to_dict_for_mongo'
ME: added a test to check for '_duration' property in 'to_dict_for_mongo' in parsed_instance
ME: added missing '_duration' property
ME: changed return value to ''
ME: added missing '_duration' property and its value
ME: added missing '_duration' property and its value
ME: changed '_duration' value to ''
ME: updated fixtures
ME: added 'DURATION' tag to header list
ME: updated test with '_duration' key and its value
ME: added a check for strings that don't match date format
ME: added 'calculate_duration' function in timing lib
ME: refactored 'get_duration' function; moved calculation method to timing lib
ME: updated xml instance files
ME: added a test for valid start time and invalid end time
ME: removed ternary operator
ME: return empty string when START_TIME/END_TIME not found; prevents tests from failing
PLD: remove extra newline
Merge pull request #724 from onaio/676-support-for-additional-metadata

676 support for additional metadata