LDAP STARTTLS option.

Summary: For GitHub PR 246.

Test Plan: Looked at the option.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4485
Added support for STARTTLS with LDAP

New config value 'ldap.start-tls' (defaults to false) for STARTTLS
support over LDAP