proof of concept: Rails 4 Queue API

As discussed in the mails, i created a proof of concept to integrate QC with the Rails 4 Queue API. You can find a sample application, with the proper setup and some examples here https://github.com/senny/queue_classic_queue_api

This is only a proof of concept. The code was not refactored and I open the PR to start a discussion around the implementation.

I created a separate file `queue_classic/rails` which is not required by default. It contains all the glue-logic to integrate with rails.

#### Rails autoload & Marshal

The autoload problems and `Marshal.load` were solved in a recent PR on rails/master.

> The current implementation has a big problem. I use `Marshal` to dump and load the job objects. The autoloading of Rails does not work with `Marshal`. This means that there are frequent `const missing` errors when jobs are queued, with object datatypes. (The Mailer job for example does not work):

```
! 	 FAIL
! 	 	 {:id=>"28", :method=>"QC::Rails::Job.run", :args=>["ActionMailer::QueuedMessage::DeliveryJob", "BAhvOi1BY3Rpb25NYWlsZXI6OlF1ZXVlZE1lc3NhZ2U6OkRlbGl2ZXJ5Sm9i\nCDoSQG1haWxlcl9jbGFzc2MMRXhhbXBsZToRQG1ldGhvZF9uYW1lOgl0ZXN0\nOgpAYXJnc1sA\n"]}
! 	 	 #<ArgumentError: undefined class/module Example>
```