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. 
If you mean  ```<groupId>org.apache.httpcomponents</groupId>```, yes such dependency already exists in the parent ```pom.xml```. But need also separate one with the testing scope.