Add phone verification using Authy

[sarmadhashmi]

Add phone verification via SMS using Authy (Twilio API).

1. User enters phone number
2. We send phone number to Authy
3. Authy sends text with code
4. User enters code
5. We send code to Authy
6. Authy accepts/rejects the code

[sarmadhashmi]

Update on this: implemented in #32.

Going to keep keep it open in case we can find an alternative: one that might be harder to implement, but ultimately cheaper.