Add a RFC2822 compliant date pattern

The pattern is defined in RFC2822 and differs from the one defined in
RFC822:
 http://www.ietf.org/rfc/rfc2822.txt

Example:
 Mon, 20 Jan 2014 18:40:18 +0100
Merge pull request #957 from vincentbernat/feature/grok-rfc2822

Add a RFC2822 compliant date pattern