Fix two email parsing issues

There are two bug fixes here:

1. This is a fix for an issue I was experiencing when parsing certain emails. It's the same issue that was mentioned on the mailing list a while back: http://sourceforge.net/mailarchive/message.php?msg_id=31144110. It has to do with Convert.ToByte() throwing an ArgumentException instead of a FormatException in some cases. It's a simple fix, and I've included a test. 

2. Another simple fix. Base64.Decode() didn't handle decoding of strings containing LF characters (it only handled the CRLF combination). Convert.FromBase64String() does, so it seems like your implementation of should too. A test is included as well.

It would be great to get these merged into the official repo.
Thanks!
Marty