Use https:// protocol for Postmark API

This introduces the 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 https://github.com/OpenBuildings/postmark/issues/7.
Improve tests based on `get_send_uri`

We are now able to mock the `get_send_uri()` method during the tests and
since it is not in a constant we could set to whatever runtime value we
need. This way we can test the wrong JSON case better.
Improve secure methods naming

The interface is now more consistent with the Symfony naming conventions.
Merge pull request #15 from OpenBuildings/secure-uri

Use https:// protocol for Postmark API