#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.
Yep, this was an unnecessary change that I left in by mistake - I had
been wondering whether to leave intervals like `delay between batches`
in an interval string format or just convert them to seconds immediately
before they get used (`delay`).