Feature/rest client

Created interfaces for Coopr Rest Client API. @albertshau , please take a look. Interfaces contain all methods, are listed in the ticket https://issues.cask.co/browse/COOPR-493.  

**Note:** I added dependency to the *coopr-server* project in to the *coopr-rest-client* ```pom.xml``` for have ability to use some complicated request classes, like ```ClusterCreateRequest```. So, I think, it would be good if we could move these request objects to the separate module, which will be accessible for all other parts of Coopr system. It could be *coopr-common* module, or something like this. Please, let us to know, what do you think about this proposal.  
Don't think there is a need for this class and its children, we can just use the classes that are already defined in the server project (with an aim to separate these out later).