Mp/ldap secure

this PR adds secure (start_tls, simple_tls) ldap support as well as cleans up the error handling around ldap failure modes. 

ping @opscode/server-team 
All errors get eaten at the top level, which cares only about connection issue or auth issue. We log the details of any failure but catch all and report only as connection error, to avoid the crash and meet what the client expects.This is useful because the most common source of errors will be configuration errors - meaning we can direct clients to the error.log and find a fair indication of what/where the trouble is.