Add zxcvbn password strength estimation

zxcvbn is a password strength estimation tool. It's not perfect, but it's practical and we can use it right now.

The authors (Dropbox) even provide an iOS library, although it is written in Objective C.

We should integrate the library from Dropbox to provide an indication of password strength in the UI when the user goes to enroll a new password. We should also maybe enforce some minimum password strength, to disallow setting really bad passwords.