Use base64 encoding for text version of emails

Some mail clients and mail brokers do not properly handle quoted-printable
encoding, used in osTicket outgoing emails. Oddly, the end mail clients
render the email with trailing equal signs (=) at the end of every line,
where it was added due to QP encoding.

References:
http://www.osticket.com/forums/forum/osticket-1-7-latest-release/troubleshooting-and-problems-aa/10708-solved-microsoft-exchange-internet-mail-lines-end-with
RFC 2045 section 6.7, http://www.ietf.org/rfc/rfc2045.txt