Feature/rest client

Start to implement Client Interfaces, submitted before according to the https://issues.cask.co/browse/COOPR-501 jira ticket.
@albertshau , take a look, please, is such structure ok for you? If you have some recommendations, please, leave these here. For now, only ```AdminRestClient``` and ```ClusterRestClient``` are implemented. I'm also going to finish with another clients and created unit tests in next two days. 
By the same reason, as you have noticed below. But, I added the same ```getAll``` method with the full URL as parameter and changed returned type for this method to the ```List``` instead of ```Set``` and then had ability to reuse ```getAll``` method.