Change print and except usages for python3 compatibility in owtf.py

+ Use print as a function instead of a statement thanks to \__future__ [\(_Ressource_)](http://docs.python.org/3.4/howto/pyporting.html#from-future-import-print-function)
+ Capturing Exception now use __as__ instead of __,__ [\(_Ressource_)](http://docs.python.org/3.4/howto/pyporting.html#capturing-the-currently-raised-exception)