Changeset 180ab31 for protocols/purple

Added some neat whatsnew code that keeps track of the newest version of
BitlBee used by a user, and if it looks like s/he hasn't used this one
before, show a list of new features that may be interesting.

Since I don't think im.bitlbee.org users will read any changelogs ever,
this is probably not a bad idea. If you hate it, the following command
should get rid of it forever: set last_version 9999999