Alternative: replace '%' style formatting with str.format()

This is basically the same as PR #52, but using the last option from https://github.com/mmb/weechat-otr/commit/21fdde8c6d510a5a50d557d29ae2fe8823953b39#commitcomment-4790741 where applicable.

I've also been a bit more generous in terms of whitespace and newlines in general and think this is easier to read than my initial proposition.