Make mail tab date parsing robust.

Example: Wed, 28 Aug 2010 18:50:04 +0200 (CEST)
This date lead to a SyntaxError.

1. Remove brackets, so that DateTime can parse this date
2. "Handle" SyntaxErrors by returng an empty string and skipping the date,
   so that the view does not raise exceptions.
Merge pull request #9 from 4teamwork/jone-mailtab-date-parsing

Make mail tab date parsing robust.