[7.0] [ADD] 'document extract from database'

Hi, 

I propose a module developped by Julius Network Solution, I got here http://bazaar.launchpad.net/~julius-network-solutions/julius-openobject-addons/7.0/files.

I did some refactoring (flake8, Pylint, adding tests)

Coverage: 93%.

Note: Travis fails for Pylint bugs in other modules.

Thanks for your review.
Thanks for you review. 
I'm not very fluent in english. Are you agree with that: 
* Module Name: document_extract_from_database --> document_export_from_db_to_fs
* Name: Extract Documents from database --> Export Existing Documents from Database to File System; 
* Description replaced by: Add a Button to export existing documents from database to File System