Builder for events request

[OndraM]

Only item-property support is implemented in the builder (interactions and user merges are missing so far - will be implemented later).

Example usage is similar as in #15:

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

$response = $matej->request() // not yet implemented in the Matej class
    ->events()
    ->addItemProperty(ItemProperty::create('1337', ['valid_from' => time(), 'title' => 'Title']))  // add just one command
    ->addItemProperties([
        ItemProperty::create('666', ['valid_from' => time()]),
        UserMerge::targetUserFromSourceUser('target-id', 'source-id'),  // command not yet implemented
        Interaction::view('user-id', 'item-id'),  // command not yet implemented
    ])  // add multiple commands
    ->send();

[foglcz]

I love it!
Beware of possible API changes if master api sig gets changed thanks to
#15 (comment)