[FEATURE] Searching for yubikeys in LDAP

Hello,

I needed to search for yubikey prefixes in my LDAP directory (which is more convenient than populating yubikeys manually and more secure than allowing auto-provisionning), so here is the pull-request. :)

It can be activated by setting LDAP_YUBIKEY_ATTRIBUTE to a valid LDAP attribute containing yubikey prefixes (be sure to have LDAP properly configured).

It works by binding the LDAP (the same way used for authenticating users) and looking up for this attribute.

Let me know if I can improve this patch in any ways.

I'll have a look at Yubiadmin to add this setting to the admin interface too, I think I'll be able to provide a pull-requesst by the end of the week.

Thank you for your great tools !

Regards,