implemented res.location()

So far only res.redirect() supports URL-logic like for mounted apps. When setting the location header manually you don't get all that stuff, when you don't want to redirect. This is useful for writing REST apis.

I implemented res.location() to determine the url and set the header. res.redirect() now internally uses res.location().

The only thing I'm not sure about is about how `res.redirect()` gets the url from `res.location()`, that's kinda ugly. Should this logic be refactored out into a seperate function? Like:

```js
app.smartUrl = function (url) { /* ... */ }
```