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.

@maethu please take a look