use SimpleSAML_Auth_LDAP class for handling LDAP access; this aligns handling of the "attributes" parameter with other LDAP auth variants using SimpleSAML_Auth_LDAP, but breaks backwards compatibility with login-ldapmulti itself (as the "attributes" value was actually used as an LDAP filter, eg. "objectclass=*")

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@825 44740490-163a-0410-bde0-09ae8108e29a