Rack App

Some people may desire more customization over the application integration than the existing Rails engine affords, but currently there's no way to opt out of the engine, since it's always loaded when `Rails` is defined.

This change modifies the load sequence to prevent the engine from being automatically required. This breaks backwards compatibility, but updating the `Gemfile` will keep the existing functionality:

``` ruby
gem 'rapporteur', require: 'rapporteur/engine'
```

It also adds a Rack app can be mounted into Rails via the routes:

``` ruby
get '/status', to: Rapporteur::App.new
```