Yubikey

Some important issues:

1. This new feature requires php PEAR to be installed in the system. Fortunately it is not necessary if the feature is not going to be used.
2. In order to activate the feature for a server Web API client's id and key must be configured in conf/sysconf.php.
3. I had to move some stuff around related to Google Authentication. This feature is not working for me so I couldn't test if I broke something and we have no regression tests. I guess it would be a good idea to test Google Auth before proceeding with the merge, just in case.