feat(authentication): add application authentication

Changed BillomatClient::__construct signature:
  - add optional parameter $apiAppId
  - add optional parameter $apiAppSecret
  - moved optional parameter $version to the last position as it is not set normally

This change allows the developer to use billomat’s application based authentication and thus higher and completely separated rate limits.

BREAKING CHANGE: BillomatClient::__construct signature has changed

To migrate the code follow the example below:

Before:

// BillomatClient::LATEST_API_VERSION needed to be set explicitly when the fourth parameter is in use
new BillomatClient('billomat-id', 'api-key', BillomatClient::LATEST_API_VERSION, true);

After:

// not using application authentication
new BillomatClient('billomat-id', 'api-key', null, null, true);
// using application authentication
new BillomatClient('billomat-id', 'api-key', 'app-id', 'app-secret', true);