Added empty default option for appname

> Usually, when you change an
> application name, the agent simply discards the current transaction and does not
> send any of the accumulated metrics to the daemon.
https://docs.newrelic.com/docs/php/the-php-api

See related discussion in https://github.com/ekino/EkinoNewRelicBundle/issues/24. Adding the xmit parameter as another configuration setting would an idea.

Also some smaller improvements.