Allow overriding of controller

I had to skip some filters in the controller to get `better_excepton_app` running. This is necessary, as it inherits from `ApplicationController`.

According to http://edgeguides.rubyonrails.org/engines.html#overriding-models-and-controllers the recommended way to go is concerns. Therefore, I moved all logic of `HttpErrorsController` into a concern.

This is how my `HttpErrorsController` looks like now. It's located under `/app/controllers/better_exception_app/http_errors_controller.rb`.

```ruby
require "better_exception_app/concerns/controller"

class BetterExceptionApp::HttpErrorsController < ApplicationController
  skip_before_filter :check_for_current_tenant
  skip_before_filter :authenticate_user!
  skip_authorization_check

  include BetterExceptionApp::Concerns::Controller
end
```

I was not able to run the automated tests, because of eric1234/test_engine#1.

Another thought: It might not always be necessary to do `<%= link_to 'Log-out', logout_path if respond_to? :logout_path %>`.
According to http://edgeguides.rubyonrails.org/engines.html#routes appending `main_app` like `<%= link_to "Home", main_app.root_path %>` is also a good way to access the route helpers of the main application.