nullprpl: pidgin null protocol plugin

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Nullprpl is a mock protocol plugin for [Pidgin](http://pidgin.im/) and
[libpurple](http://developer.pidgin.im/wiki/WhatIsLibpurple). You can create
accounts with it, sign on and off, add buddies, and send and receive IMs, all
without connecting to a server.

Nullprpl was [added to Pidgin on
5/30/2007](http://developer.pidgin.im/viewmtn/revision/info/17c1e71f633d40fd14e46c08a17a500605d793c8)
and debuted in
[version 2.0.2](http://developer.pidgin.im/viewmtn/revision/file/1c5a391126b5b506a29f0b9e34160d239cb783c6/ChangeLog).
See the [Pidgin ticket](http://developer.pidgin.im/ticket/811) and the
original [SourceForge
tracker](http://sourceforge.net/tracker/index.php?func=detail&aid=1033153&group_id=235&atid=390395)
for more. Nullprpl was originally distributed as a stand-alone protocol plugin;
the last release was [mockprpl 0.3](mockprpl-0.3.tar.bz2).

Beyond basic buddy list and IM functionality, nullprpl supports presence and
away/available messages, offline messages, user info, typing notification,
privacy allow/block lists, chat rooms, whispering, room lists, and protocol
icons and emblems. Notable missing features are file transfer and account
registration and authentication.

Nullprpl is intended as an example of how to write a libpurple protocol
plugin. It doesn't contain networking code or an event loop, but it does
demonstrate how to use the
[libpurple API](http://developer.pidgin.im/doxygen/dev/html/group__core.html#_details)
to do pretty much everything a prpl might need to do.

Nullprpl is also a useful tool for hacking on Pidgin, Finch, and other
libpurple clients. It's a full-featured protocol plugin, but doesn't depend on
an external server, so it's a quick and easy way to exercise test new code. It
also allows you to work while you're disconnected.