Class override in ApiResponseObject

Now it is possible to specify class for ApiResponseObject:
```
@ApiResponseObject(Integer.class)
```
or
```
@ApiResponseObject(City.class)
```

Should be possible to annotate old-school servlets:
```
@ApiMethod(path = "/implicitResponseObject", verb = ApiVerb.GET, description = "a-test-method")
@ApiResponseObject(City.class)
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
...
}
```