Changed displayName of user_ldap app to use template instead of single attribute

As i'm authenticating against a foreign LDAP server and this server does not provide a good display name, i crafted this patch, which allows to use a template in the display name field of the ldap_user configuration.

The template allows to specify references using the ${attr} syntax. Each "attr" is resolved using an LDAP request and replaced in the template.

English documentation is updated as well.