Merge pull request #69 from mgorny/otp-ldap

Proper bind for OTP setup
login: give a proper info() message when asking for strong auth.
login: ask only for password when upgrading auth.
Add more docstring on okupy.common and okupy.otp.
Revoke OTP tokens in views rather than backends.

Since we use a single revoked token database, we should also revoke each
input once. When revocation was done in models, the second backend would
always hit the token already revoked. Instead, revoke them in views.
Use shorter otpauth:// URI.

Since we're going to be the auth backend for whole 'gentoo.org',
and nothing else uses OTP there, we can just use 'gentoo.org'.