Updated callback strings to comply with libpurple.

FIXES:
 - Updated callback strings (replaced underscores with minus symbol).
 - Now all UiOps callbacks are added using add_callback(type, name, func).
 - Updated changes on nullclient-ecore.py.
 - Added receiving-xmlnode example on nullclient-ecore.py.
 - Removed deprecated nullclient.py

Signed-off-by: Bruno Abinader <bruno.abinader@indt.org.br>

git-svn-id: https://garage.maemo.org/svn/carman/branches/carman-0.7-beta2/python-purple@1359 596f6dd7-e928-0410-a184-9e12fd12cf7e