Bug Fix & support email addresses enclosed in multiple brackets

Fixed Bug in Rfc2822DateTime with CustomDateFormats. They weren't working with string timezones (e.g. PST) due to an error I'd made when writing it last week.

Allow email addresses to be enclosed in multiple brackets, so 
```
"First Last" <<first.last@email.com>>
```
will work, being treated as 
```
"First Last" <first.last@email.com>
```
This may seem like an odd thing to add, but I'm adding it because the system I'm using OpenPop in has received some emails with addresses like this.