Add StartTLS connection support

```js
/*
 * LDAP connection with StartTLS
 */
var ldap = require("LDAP");

var l = new ldap({
	uri: "ldap://ldap.my.domain",
	starttls: true,
	binddn: "uid=proxy,dc=my,dc=domain",
	password: "mypassword",
});

l.open(function(err) {
	if (err)
		console.log("l.open() => error=" + err);

	l.close();
});
```