WIP - First Celery implementation

Hej PR reviewers!

A few comments, on this PR. First of all, it is a WIP, so it still needs some work, but I think that is better that you start reviewing this piece of code.

It is working though :-):

* ```ngi_server.py``` will create a Celery worker that will listen to a RabbitMQ queue called ngi_pipeline in the virtual host bionextgen_production, easy peasy.
* There is only one task defined now, that basically will trigger the analysis.
* The file ```illumina_finished_msg.py``` is just a testing file for the messaging system now, you can see there an example of how to send a message to Celery.

I have tested everything in my laptop, hopefully configuring production will be just slightly more complicated... :-P

You can merge this if you think it's right, I'll open some issues and do smaller PR.

@guillermo-carrasco Is decorator just to avoid writing __enter__ and __exit__ because those would be blank in this case or does it do something fancier?