0.9 compatibility: rename from .inc -> .php
0.9 compatibility: Move authentication methods

Starting with 0.9, simpleid has a hook_user_verify_credentials()
function that can be provided by an extension. Let's use this function
instead of creating our own store augmenting the filesystem store.

This also allows us to move the Auth_Yubico class into the extension's
subdir.
0.9 compatibility: change verification parameter

hook_user_verify_credentials now gets string $uid as first parameter, no
longer array $test_user, which was the result of user_load($uid).
0.9 compatibility: Adjust CSS to match front page
Allow login without user name

Uses the default filesystem user store to automatically determine the
correct user name from the given yubikey OTP.