#8 Don't generate new keys for semantically identical usernames

[Nick Mathewson]

In many IM protocols, it's possible for one username to
be written in multiple ways. In AIM, for instance,
"Alice" can be written as "alice" or "AlIcE" or "Al
Ice". When looking for a key, however, OTR only finds
it if the username is identical to the known version.
(I've confirmed this for private keys; it may be true
for public keys too.)

It would be good if OTR could match usernames despite
cosmetic differences in format.