Added an handler for NewRelic

* implemented a test which verifies that the fallback handler handles records when the NewRelic PHP extension is not installes
* implemented the actual handler

There are a few problems that Id like to discuss:
* how to properly test it? The `NewRelic` extension is proprietary, so you cannot obtain it for free
* do we need the fallback handler? It would be super useful for local development, since developers dont usually get the NR extension on their local machines - at the same time I recognize that there is no fallback handler for other handlers (ie `Graylog2`)
* currently, that test seems a little bit too weak to be called a "test"
* if we use `function_exists` we could add some more meaningful tests, but at the same time `extension_loaded` is more *right*