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. 
This constructor is needed to avoid a creating of instances from this class, because this class includes only static method(s).
You could take a look http://www.informit.com/articles/article.aspx?p=1216151&seqNum=4