Use https:// protocol for Postmark API

This introduces these new methods:
- `Api::get_is_secure()`
- `Api::set_is_secure()`
- `Api::get_send_uri()`

They could be used to get/set the secure flag and get the send URI based on that flag.

The new default is to use the `https://` protocol for the Postmark API.

You could use the previous default `http://` with:

    $transport = Swift_PostmarkTransport::newInstance('your api key');
    $transport->api()->set_is_secure(false);

Resolves #7.