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.

I've also been a bit more generous in terms of whitespace and newlines in general and from the feel of it I'd say this matches the general coding style of the rest of the code a lot better than my initial proposition.