Allow callable objects as the location

One thing I frequently add to my projects is the ability to use callable objects as the location.

Things like this:

```ruby
class MyController < ApplicationController
  def create
    @thing = Thing.create(thing_params)
    respond_with @thing, location: -> { some_route_path(@thing) }
  end
end>
```

This `Responders::LocationResponder` does exactly this.