676 support for additional metadata

closes #676 
this function should be moved outside the class or to a library, maybe the [timing lib](https://github.com/onaio/onadata/blob/master/onadata/libs/utils/timing.py) so that it can be reused.

Also, can you add a test for a valid start date but invalid end date to test that return an empty string on a `ValueError` doesn't cause problems?