LDAP: use OC_Cache to cache results from LDAP. Default is set to 10 min. Should improve performance especially when LDAP users use the sync client, because userExists checks with the LDAP server are reduced.

LDAP: check if php-ldap is installed. If not, give an error output. FIX: blank Users page when the module is not installed.

LDAP: split up LIB_LDAP into Access for LDAP interaction functions and Connection for configuration and resource management. Adjust user_ldap, group_ldap and the app accordingly.

LDAP: switch user backend from extending class OC_User_Backend to implementing OC_User_Interface for improved flexibility

This reverts commit 3937d19fa2ef3da2719d0425ce02707d0345960c.