Support for the l2vpn vpls family

this is the same PR as https://github.com/Exa-Networks/exabgp/pull/134 but against -dev version of exa.
debug output can be found here: https://gist.github.com/tehnerd/a088a3b2c6c68c1b6aa7
As now two functions are using Logger() in this file, should we make logger a global and only initialise it once ?
It would go around the issue that Logger() can not be called in the global scope and would surely improve preformance slightly. ie : 

logger = None

def ... ():
   if logger is None:
     logger = Logger()

in both functions.