Campaign request builder

[OndraM]

Fixes #38.

Example usage:

$matej = new Matej('accountId', 'apikey');

$response = $matej->request()
    ->campaign()
    // Request item sortings
    ->addSorting(Sorting::create('user-id', ['item-id-1', 'item-id-2', 'item-id-3']))
    ->addSortings([/* array of Sorting objects */])
    // Request user-based recommendations
    ->addRecommendation(UserRecommendation::create('user-id', 10, 'emailing', 1.0, 3600))
    ->addRecommendations([/* array of UserRecommendation objects */])
    ->send();

[foglcz]

Personally I'd add an integration test as well - can be done after merge of #40
(integration tests weren't available at the time of PR creation)

[OndraM]

Also we need to wait for RAD-719 with integration tests :)