Added an optional dependency on SleekXMPP

Will be necessary for XMPP bundles
Flush I/O Handler output at the end of shell.execute()
Added basic classes to make an XMPP bot

Based on SleekXMPP
Added an XMPP shell interface

- XMPP Shell is an XMPP client, answering to chat messages
- Added an EventData type to pelix.utilities