Openldap improvements rebased

Openldap collects a bunch of interesting metrics about its internals and the queries it is answering, and exposes them in an LDAP database which any LDAP client can query. This plugin acts as such a client, grabs most of the numerical values and submits them into collectd.

More details about openldap's `cn=Monitor`: http://www.openldap.org/doc/admin24/monitoringslapd.html

![openldap](https://cloud.githubusercontent.com/assets/37533/4090833/d6b5b7a2-2f81-11e4-9c4a-fb98a1f192d0.png)