add proper SMTPUTF8 handling for MUA mode

[jetmore]

#20 is adding more of the test mode - saying use SMTPUTF8 and error if it's not available. For MUA mode this should be more conditional - see if it's needed based on the content of the message and make sure everything is encoded properly

note also 8BITBODY, which appears to be complementary to SMTPUTF8 but which we don't currently support