Make it possible to use without twilio

[Rolf Erik Lekang]

Since TwilioBackend is imported in nopassword/backends/__init__.py. It will crash without the twilio package.

Possible solution in nopassword/backends/__init__.py:

# -*- coding: utf-8 -*-
from .email import EmailBackend
try:
    from .sms import TwilioBackend
except ImportError:
    pass # twilio package not installed