#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.
This is due to the kind of clunky way that I handled interval processing for
backwards compatibility (and speed). Everything gets coerced to a
number of seconds - but defaults don't get coerced, so they have to be in
seconds...