Added read-only api key as an option for progress requests

For security reasons we do not want to send an api key as part of the url for any requests to Zencoder. On enquiring about this with them we were informed that you can create a read only api key - which can only be used for things like progress requests, and not for creating jobs. This means that even if someone gets hold of it they are unable to abuse our account, merely to try and monitor what we are doing. If this read only key is not set in the constructor then the code behaves as before, using the main api key.