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 also used ```StringUtils``` from this lib, but I investigated that I can replace it by the ```Strings``` from the ```guava``` lib. But, anyway, in the parent ```pom.xml``` file there is apache commons dependency:
 ```<dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.3</version>
        </dependency>
```
so, I left ```UnsupportedOperationException``` exception.