Add logging functionality

[jcass77]

I think it would be useful if stormpath-flask logged important events (e.g. new user created, passwords changed, login / logout, etc).

Users of the stormpath-flask extension could then set the logging level and add an appropriate handler with something like:

logger = logging.getLogger('stormpath')
logger.setLevel(logging.WARNING)
handler = RotatingFileHandler(my_app.config, mode='a', maxBytes=1000000, backupCount=3)
logger.addHandler(handler)