NewRelic handler: added the app name

In New Relic, you can specify different apps such as **solr.packagist.org**, **packagist.org** and so on.

This PR allows you to set the appname for the log by just adding a `$record['context']['appname']`.

@seldaek I think I can throw custom exceptions from the new relic functions in the tests to actually verify that they are throw, let me know. This is a backward compatible change, would be great to see it included in **1.6**.