remove export array indexing by identifier
  non-integer indexes causes json_encode to assume the
  associative array is an object which will cause the
  entire array of exported objects to be merge into
  one giant object
improve logging in BaseSchameMapper
refactor ExportCommand, improve usage and logging
  refactor general helpers into CommandUtils
  allow multiple data type export
  add [--create-zip|-z] option to generate timestamped zipped output