ServerAPI: merge srvSetResponseCode, srvSetContentType and srvWriteBuilder methods

I'm currently trying to write a `ServerAPI` instance for `MonadHandler`, the Yesod monad. There's one problem: In Yesod, there isn't a function for setting the response status code! There are functions for sending the body of the response that take a status code as an argument, but there isn't one function that just sets the status code like there is in Snap. (One possible workaround would be to wrap the Yesod monad with a `StateT Int` transformer, but I don't want to do that.)

This pull request replaces the three methods `srvSetResponseCode`, `srvSetContentType` and `srvWriteBuilder` in `ServerAPI` by one single method

```haskell
srvSendResponse :: Int -> BS.ByteString -> Builder.Builder -> forall a . m a.
```

that takes the status code, the content type and the content of the response. This API interface assumes less about the architecture of the web server library monad (like the ability to set the status code of the response), hence it should be easier to implement this one function than all three.