Custom Mount Route + Engine-based Routes + Merge NewRelic patch + Mongoid 3 + Memcached + 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

8) Upgrade `MongoidCheck` to support Mongoid 3 with fallback to existing Mongoid 2 logic

9) Add Mongoid query syntax for DelayedJob check

10) Add `CacheCheck` class to check memcached (should work with both `Dalli` and legacy `Memcached` gem, because it uses `Rails.cache`)

**NOTE:**
For some reason, Rails autoload doesn't like the `OKComputer` namespace and I found I had to alias `OkComputer` (lowercase 'k') to `OKComputer`. Following the Rails camel-to-unscore conventions I'd propose to change the app namespace to `OkComputer` / `ok_computer` (with a deprecated alias to old uppercase-k namespace), but I have not done it in this PR