Revised New Relic daemon logging approach to go to file instead of stderr.

Update the newrelic logging approach to include both the Agent and Daemon logs.

The agent writes to stderr.

The daemon writes to a well-known log file and then a .profile.d script is created
which tails the output of this file to stderr.

This approach enables 'heroku logs' to get all the New Relic logging for *both* web
and worker instances. 

Finally, introduced more system INI variables and documentation in the
ext-newrelic.ini and corresponding environment variables (with defaults) in the
newrelic.sh .profile.d script.

@dzuelke