Initial support for SSL/TLS connections.

This provides SSL/TLS upgrade without requiring the native libpq bindings (see issue #25).

Certificate verification is done against the default list of CA certificates used by [`tls.connect`](http://nodejs.org/api/tls.html#tls_tls_connect_port_host_options_secureconnectlistener) ("*several well known "root" CAs will be used, like VeriSign*").

Name verification also seems to work (as performed via `tls.connect`), thereby making it an equivalent to libpq's `sslmode=verify-full`.

It would need further enhancements to be able to pass [other SSL/TLS parameters, via `tls.connect`](http://nodejs.org/api/tls.html#tls_tls_connect_port_host_options_secureconnectlistener).