support ACTIONs (/me) in otr messages
add [nick] prefix to message

if there is no query buffer, info_get_('irc_buffer') will return the
corresponding server buffer. we need to add a prefix here to avoid
confusion.
allow smp question and answer to contain non-ascii
print a warning when a message was not sent

if require_encryption is set and no otr tunnel is established, give a
warning that the message was not actually sent.
replace '%' style formatting with str.format()

- this commit should not change the behavior of the code in any way
- i've used the named syntax everywhere except when adding names would
  only have complicated the code without increasing readability, e.g.
  `'len {}'.format(len(msg))`

fixes mmb/weechat-otr#29