#119: accept suite.rc interval configuration in ISO 8601 formats

This changes the syntax for specifying intervals of time in the
`suite.rc` file to use ISO 8601 interval syntax.

@hjoliver, please review.
I've reverted it, but this is an interesting point - I would probably go for:
```python
            cfg_event = self.config.cfg['cylc']['event handler submission']
            self.eventq_worker = event_batcher(
                'Event Handlers', self.event_queue,
                cfg_event['batch size'], cfg_event['delay between batches'],
                self.suite
            )
```
which wouldn't get any complaints from [pep8](https://pypi.python.org/pypi/pep8).
However, I think the longer line is better for grepping quickly.