print an error when we try to connect an inexisting server instead of crash

try to fix ticket 148. has some problems: it wont display the accounts with erros in the accounts window. Only after you close it and open it again

service discovery is now asynchronous, we can send and receive other messages while we browse agents (YEAH !)

update copyright
add missing headers

use getTimestamp function to know the time the message has been sent (especially in case it hase been stored on the jabber server when user is offline)

New agent Browser (JEP 30 complient)
Upgrade of french translation

do not create a new connexion in the core if it already exists

save events when no visual plugin (like gtkgui) is registered and restore them when such a plugin resiters

gtkgui plugin ask roster when it starts (in case an account is already conected)\nthread in started in plugin.py

new QUIT signal : we can or not close the core

runCore can now recieve the -c option to be lunch as a client. In such a case, it connects to a gajim server (a sock plugin) and not to a jabber server