CiviMail: Wrap long lines in the generated emails

RFC 2821's point 4.5.3.1 requires that emails do not have lines longer than 1000 characters, and CiviMail should honour this, given that some SMTP servers truncate longer lines (rightfully so).

As something along an email's route can recode it (in theory, recoding a Chinese email full of UTF-8 characters recoded to 7-bit quoted-printable could extend a line's width by a factor of three or four), we should wrap long lines at the earliest space past two-hundredth character.

The relevant forum thread: http://forum.civicrm.org/index.php/topic,3347.msg15077.html#msg1507