Two string changes for translation in Software Center

[Gary Lasker]

Hello Translators!

We recently received a bug report from Dennis Baudys that pointed out a
non-localizable date string in the software item screen of Software
Center (thanks, Dennis!):

  https://bugs.launchpad.net/bugs/751068

We definitely wanted to fix this one with the result that we have two
string changes for translation as shown:

 1. 'Purchased on %s' has changed to 'Purchased on %Y-%m-%d'
 2. 'Installed on %s' has changed to 'Installed on %Y-%m-%d'

Note that we've included a comment to the translator in the .pot file
for each of these strings as well.

My apologies for this late string change!  But I think the result will
be a much nicer presentation of these date strings.

Thank you for your good work!
Gary Lasker