Refactor XMPPWSGI server: split XMPP driver interface from server logic.
Introduced XmppWsgiServerInterface class which should be used to implement interface to additional XMPP libraries and to allow XmppWsgiServer communicate with them.
Also this tricks helps(should) with testing due to we could easily be abstracted from any XMPP realisaion (see MockXmppWsgiServerInterface) and wouldn't bind XMPP library and XmppFlask app.
Additionaly, XmppWsgiServer environ has extended by new variables: - APP_JID: XmppFlask app bounded full JID. App should know his own JID. - APP_MUCNICK: XmppFlask app MUC nick if message type is groupchat. - XMPP_ID: XMPP stanza id. If not specified by income event, it would be created by using 'xmppflask' as prefix and uuid1 first 8 bytes as suffix. - XMPP_DELAY: Shows delay in seconds between current UTC time and point when income event was occurred.