Quoted-Printable: allow up to 76 characters per line

This conforms to the QP specification [1] -- page 22 says that 76 characters,
not including the CRLF, is OK.

Please note that the word wrapping code in Imap::wrapFormatFlowed (Encoders.cpp)
tries to collaborate well when it comes to Unicode and UTF-8 by wrapping at 72
characters if possible. It might be interesting to change *that* code to encode
into UTF-8 and check the line length in bytes in future, but I don't find that
ciritcal -- this pre-wrapping at the time of format=flowed production is meant
to only reduce the need for "ugly" excessive line break during the Q-P phase.
Messages which benefit from a correct guess about the UTF-8 byte size are by
definition going to be Q-P encoded, and hence not that much readable when viewed
in their raw form.

[1] http://tools.ietf.org/html/rfc2045#section-6.7
format=flowed: allow up to 76 characters per line

See the inlined comments for reasoning.