MIME Email message class

Filenames of attachments are now encoded if they contain non-ASCII characters.

The default mailer identity was reset to an empty string. This prevents the generation of the X-Mailer header and avoids an issue with mail servers confusing messages sent by the class with spam.

The main class can now cache individual message parts that attach or embed files in messages. When sending personalized mailings to many recipients for whom such file parts do not change, this avoids the overhead of encoding those message parts again. The SMTP driver sub-class was enhanced to disconnect from the SMTP server after a given number of consecutive deliveries in bulk mail mode in order to avoid exceeding the limits imposed by certain SMTP servers.

The generation header values were improved to make
them clear by avoiding using q-printable encoding
on headers that do not need it.

The main class and the SMTP class were fixed to allow sending messages without a To: header as long as they have Cc: or Bcc: headers. The sendmail delivery class was fixed to remove a duplicate variable declaration.