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. 
I created and used this exception by analogy with rest clients, we implemented in the cdap project. Also there are some similar exceptions in the ```javax.ws.rs``` package (like ```BadRequestException```, ```NotFoundException``` etc) and these exceptions are also runtime. 
But if you want, I could change it to the checked exception.