Rfc2822DateTime Invalid Formats

Changes the behavior of Rfc2822DateTime for invalid date formats to fail gracefully by logging the error and returning DateTime.MinValue.
In addition the ability to specify custom formats has been added by setting Rfc2822DateTime.CustomDateTimeFormats.

The motivation for this change was issue #16 (a server that was regularly sending me emails was using an invalid date format and these messages couldn't be downloaded with OpenPOP)