Added support for writing static response headers to HTTP actions.

Awhile ago I noticed that there wasn't any documentation on how to set custom response headers in rest-core and made note of it in issue #59.  I was unaware at the time that there wasn't a facility for setting custom response headers.  Since I need this capability to support CORS, I've added a simple static header configuration that can be applied to any Handler, as shown below.  The tooling that I have submitted does not allow you to set headers based on the input environment or on the generated output, but I have been unable to come up with a situation when that would be warranted.  Though if there is interest in that functionality I am happy to look into what it would take to accomplish.

```haskell
get :: Handler WithInfo
get = defHead $ mkHandler (jsonO . someO) $ \e -> do
        i <- ask
        return $ SampleRes i

defHead :: Handler m -> Handler m
defHead = setResponseHeaders [("Access-Control-Allow-Origin-Test", "localhost:3000")]
```