Use https:// protocol for Postmark API

[hkdobrev]

This introduces these new methods:

• Api::is_secure()
• Api::set_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_secure(false);

Resolves #7.

[hkdobrev]

@jaketoolson Since you are the one who suggested #7, what do you think about that?

[hkdobrev]

Updated the interface with better naming.

/cc @dkyosev

[dkyosev]