Making New Relic a configuration option in the app script.

Currently, enabling New Relic on a server requires modifications
to the app.js script. This makes that not necessary. This change
does nothing to actually _set up_ New Relic on the server,
which, if desired, should be a different issue against the
cookbook repo.

Fixes #432
Moving the contents of newrelic.js into local.ex.js.
Adding comments for the new relic fields.
Merge pull request #457 from DavidEBest/devel-newrelic

Making New Relic a configuration option in the app script.