Add hal explorer for conveniently inspecting rest apis

The hal explorer is like swagger for HATEOAS. 

https://github.com/toedter/hal-explorer

All that is needed is to add this dependency:

```
     <dependency>
         <groupId>org.springframework.data</groupId>
         <artifactId>spring-data-rest-hal-explorer</artifactId>
      </dependency>
```