Default TLS ServerName to the host in the DSN.

A TLS configuration must either have a ServerName or specify
InsecureSkipVerify. In most cases, the ServerName value will
match the host part of the address in the DSN. This change
updates the DSN parser to default the ServerName to the host
value provided unless InsecureSkipVerify is specified.
Merge pull request #283 from metcalf/am-default-servername

Default TLS ServerName to the host in the DSN.