Adds recipe to update the date/time format of the clock in the menubar

The default format causes the clock to look like 'Tue Mar 18 8:22:07 PM' and
there is an attribute sprout->settings->clock_format to allow overriding.
There is also a link in the attributes file as to where to look for the
formatting options.
Merge pull request #241 from codeword/clock

Adds recipe to update the date/time format of the clock in the menubar