Custom Mount Route + Engine-based Routes + Merge NewRelic patch + Cleanup

1) Delete unneeded auto-generated Rails files in `/app`, `/script`, and `/spec/dummy` dirs, and cleanup `/spec/dummy` code.

2) Use `mattr_accessor` to reduce code in `configuration.rb` (no change to function)

3) Convert `routes.rb` from `Rails.application.routes.draw` to `OkComputer::Engine.routes.draw`. The engine is now automatically mounted at `/okcomputer`, and you can change this via a new `OkComputer.mount_at` option.

4) As part of #3, I'm pretty sure we need to namespace `OkComputerController` so it is now `OKComputer::OkComputerController`. This breaks the okcomputer-newrelic-ignore gem, so...

5) Merge the functionality of https://github.com/sportngin/okcomputer-newrelic-ignore into the `OkComputer::Engine` initializer. There's no practical case where a NewRelic user would want NewRelic to count OkComputer.

6) Get rid of RSpec deprecation warnings in specs

7) Add test case for Delayed::Job#size using mocks