Documentation incorrectly claims that dates are parsed with python-dateutil

[djneades]

The tarsnapper Readme.rst states:

It [tarsnapper] will parse the date using the python-dateutil library, which supports a vast array of different formats,

However, the tarsnapper source shows it to be using datetime.strptime, with a couple of pre-canned formats and the ability to specify a date format on the command line.

This difference is significant, as dates that one might expect to be correctly parsed are actually rejected.