Added set_alias to Buddy class.

FIXES:
 - Added set_alias to Buddy class.
 - Updated get_buddies_online() from Account class to return a list of
   Buddy classes instead of a tuple (name, alias).

Signed-off-by: Bruno Abinader <bruno.abinader@indt.org.br>
Acked-by: Ragner Magalhaes <ragner.magalhaes@indt.org.br>

git-svn-id: https://garage.maemo.org/svn/carman/branches/carman-0.7-beta2/python-purple@1443 596f6dd7-e928-0410-a184-9e12fd12cf7e