CDAP-412 REST endpoint to expose Service methods

Endpoint: ``/apps/{app-id}/services/{service-id}/endpoints-exposed``

Example output: 
```
[
    {
        "method": "GET", 
        "path": "/v1/product/{id}/catalog"
    }
]
```


@chtyim
regarding: https://github.com/caskdata/cdap/pull/545#discussion_r19260498
I didn't mean to transform the spec during the http request. I had meant that perhaps the service spec should be formatted differently from the beginning (a service has a set of endpoints, and shouldn't be concerned with individual handlers, each with their own spec).