Added search-and-bind feature for the core LDAP authentication plugin.

With this plugin you can:
* authenticate to a directory using a "Limesurvey username" different from the user LDAP entry RDN. For instance you can login with an email address while the true user RDN is in fact a UID
* authenticate to a directory where user-entries are spread in subtrees (when the LDAP directory requires you to bind with your full DN as you identication).
==> This kind of of authentication is known as search-and-bind LDAP authentication.

Of course the plugin remains compliant with teh previously implemented simple-bind authentication scheme.