Add support for IPv6 connections

Currently, eldap assumes that only IPv4 will be used. This change
enables callers to use IPv6 by including the [inet6] option in the
eldap:open/2 options list. This adds inet6 to the gen_tcp or ssl
connect.  For backward compatibility, [inet] is the default if inet6 is
omitted.

(This replaces pull request #110).