allow timestamps in Grouper enabled/disabled dates

From Scott Koranda

A review committee is flying into Hannover to meet at the
Albert Einstein Institute at the University of Hannover.

The members of that committee should be given permissions to
access the web site set up for the review beginning on Monday
at 10:00 AM local time Hannover. The permissions should expire
on Wednesday at 5:00 PM local time Hannover when the review is
complete.

Sitting here in Milwaukee I would like to specify the
beginning time as something like this:

2011-07-18T10:00+02:00

and the ending time as

2011-07-20T17:00+02:00

The "+02:00" is because Hannover is two hours ahead of UTC/GMT
during July.

Note that the Grouper service runs in Pasadena, which is
-07:00 or 7 hours behind UTC/GMT.