Add support for api v2 with same interface

[inakiabt]

As default, it uses v1 as always:

    Hippy::init('123', 'hippy', 'Rob');

    Hippy::add('test 1');
    Hippy::addHtml('test 2');
    Hippy::add('test 3');

    Hippy::go();

Now, you can use v2 as a new Transport:

    $v2 = new rcrowe\Hippy\Transport\APIVersion2('54321', 'cog', 'vivalacrowe');

    Hippy::init('123', 'hippy', 'Rob', $v2);

    Hippy::add('test 1');
    Hippy::addHtml('test 2');
    Hippy::add('test 3');

    Hippy::go();