Register response content converters

Add mechanism to allow for automatic conversion of response content and
content type based on user defined test conditions.

I want to do this so I can do something like the following in my setup:
```php
$app->registerContentConverter(
    function($content) {
        return $content instanceof Hal;
    },
    function($content) {
        return $content->asJson();
    },
    'application/hal+json'
);
```
And in my routes I can do this:
```php
$app->path('/', function($request) use($app) {
    $hal = new Hal('/');
    // snip
    return $hal;
});
````

The default behavior of converting Arrays to JSON has been preserved by a call to `registerContentConverter` in `\Bullet\App::__construct`