Refactored file structure to allow app to be mounted into a Rails application

As I told you in #106 I need to mount your application inside a bigger Rails project (with devise authentication). 

I've refactored some code and moved the applications code into a `KibanaApp` class. Application is the same, and the existing specs are passing. Further, `ruby kibana.rb` from top level directory still works, since I've put the app call in that file.

I've used this fork in a rails app with these lines

```ruby
# In Gemfile
gem 'kibana', :git => 'git://github.com/fabn/Kibana.git', :branch => 'sinatra-app'

# In routes.rb, this allows access to kibana only for devise authenticated users
authenticate(:user) do
  mount KibanaApp, at: 'logs'
end
```