[WO-554] use 7bit transfer encoding for PGP payload

Enigmail is unable to extract ciphertext and signature from PGP/MIME messages
for any other encoding than 7bit. See this thread in the mailing list:
https://admin.hostpoint.ch/pipermail/enigmail-users_enigmail.net/2014-August/001963.html
Merge pull request #9 from whiteout-io/dev/WO-554

[WO-554] use 7bit transfer encoding for PGP payload