Implement async starttls from ldapjs

[jduke-halls]

Generally, securing on the LDAP standard port with STARTTLS is seen as more favorable for Active Directory environments, rather than connecting to the LDAPS port.

ldapjs has implemented STARTTLS using the client.starttls as detailed here

I have created a promisified wrapper for this function to add into the project.

Thank you for all your work!

[jduke-halls]

Pull request #43