OTR integration

[Mark Doliner]

I kind of hinted at this in the devel at conference.pidgin.im MUC, but
you could maybe do some combination of the first and second options:
Have the plugin enable itself by default and have the plugin watch for
"?OTRvxx?" messages. The plugin would want to avoid altering Pidgin's
UI unless:
- The user has received an incoming "?OTRvxx?" message and has
selected "yes, use OTR"
or
- The user has manually turned on OTR. I'm not sure how this would
work, since the plugin would already be enabled. Maybe the user
generates a key? Or views the config UI?

Also, I like when dialogs have an "not now, but do ask me again later"
option. It let's me ignore the dialog for now without worrying that
I'm going to miss out on an exciting feature. Meh--not critical, and
maybe doesn't make sense in the OTR case.