fixes #33

Initial load times can be very slow on Heroku-hosted sites because Heroku spins down the dynos on sites that are not accessed very often. One solution is to use the free New Relic monitoring service, which pings your site throughout the day and prevents they dynos from going to sleep.

Simply adding the New Relic add-on is not enough. You also have to add the "newrelic_rpm" gem to your Gemfile, then `bundle install`, and add a "newrelic.yml" file in your config directory. This is what this pull request does.

You then need to set Heroku environment variables for your New Relic License Key and App Name. This is covered in the new section I added to the Readme via this pull request.

I also upgraded debugger-ruby_core_source to allow
debugger-linecache to install on 1.9.3.