Improve TLS support

We need to make a few changes to improve go-nsq support for TLS.

The default validation for TLS connections should be set to ServerName, and that should validate the broadcast_address used from lookupd, or the hostname of a direct nsqd connection against the certificate.

We also need to expose options for setting InsecureSkipVerify and the CA root so that they can be accessed via the config `Set(key, value)` interface.

cc: @mreiferson