Feature/report reaction

This pull requests implements a new style of reaction: report. Instead of denying the request immediately, the middleware only reports the status of its check. This allows the application to handle the request based on that info.

This comes in handy when someone wants to exclude certain routes from checks, e.g.:
https://github.com/padrino/padrino-framework/issues/893#issuecomment-13808244

This is intended to be used on single middlewares, e.g.:

```
use Rack::Protection::AuthenticityToken,
       :reaction => :report,
       :report_key => 'rack.protection.csrf.status'
```