ME: refactored 'retrieve' method to include 'export_handler'
ME: added list action
ME: added a test for data endpoint xls export
ME: added ability to access list action without 'pk' or when pk is 'public'
ME: replaced 'export_handler' with 'custom_response_handler'
ME: code refactor; created a separate function to handle logs
ME: refactored 'retrieve' method to include 'export_handler'
ME: added list action
ME: replaced 'export_handler' with 'custom_response_handler'
ME: code refactor; created a separate function to handle logs
ME: rendering json and xml formats only for data instance
ME: removed xml response for now
ME: refactored 'retrieve' action
ME: fixed failing tests by refactoring
Merge pull request #654 from onaio/604-add-xls-export-to-data-endpoint

604 add xls export to data endpoint