The delete usage method doesn't return a boolean result

[taojing2002]

In our design document, the delete usage method should return a boolean result. However, it return a blank string.

[gothub]

@taojing2002 the delete usage service returns an HTTP status of 200 if the request is successful, or an error status if it isn't
successful. Is that sufficient?

[taojing2002]

@gothub To me, it is fine. I just concern it doesn't match our design document -

boolean   = deleteUsage():  DELETE  /usage/:id

If we all have an agreement that we don't need to return the boolean, it is totally fine to me.

[gothub]

@taojing2002
The bookkeeper delete methods for all resources ("customers", "products", ...) all return HTTP 200 on success, so I'd say let's go with that.

I just opened issue #50 for creating a Swagger definition for the bookkeeper API. When this is created, this should be the reference for the bookkeeper API.