Don't run if newrelic extension is not enabled

Avoid fatal errors Call to undefined function
Fixed spelling of New Relic
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
Merge pull request #2 from worldskills/improvements

Added empty default option for appname