#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.
It is all subjective really - I prefer limiting everything to 79 characters, at the cost
of some awkwardness sometimes. The pep8 checker doesn't really have a check
for awkwardness, but it does for line length.

It does give an incentive to use fewer levels of indentation, which is the
right thing most of the time.