allow smp question and answer to contain non-ascii characters

Having non-ascii characters in SMP question or secret would lead to

    UnicodeDecodeError: 'ascii' codec can't decode byte 0x?? in position ?: ordinal not in range(128)

in several places of the code. This commit should catch all the cases where smp strings are passed to or from weechat/potr.

Note that the encode('raw_unicode_escape') introduced in de6de591 only appears to solve the problem, as the wrong hash is calculated (tested against pidgin/libotr).