Refactor of repository access to decouple service classes from underlying
data store

 - Added UserService and User
 - MessageService now decoupled from MongoDb and more and better unit
   tests added.
 - New integration.* packages to house all persistent storage related code
 - @Configuration classe altered in core package and new one added to
   integration package
 - Hamcrest library added to POM in core module
 - server.yaml updated with comments
Improved all READMEs in each module