7.0 add file exchange

Hi I propose this branch as PR, before accepting it we have to discuse about refactoring it in order to avoid duplicated work see #1
The aim of this module is just to add the posibility to exchange file with external system. This work do not include the way to process the file.

1: file_document : is a queue storage of document to process (as connector include a real queue management, we should review this and discuse about the best way to refactor it)
2: file_email : is a simple module that give the posibility to extract from a mail the attachment and put it into the file-document
3: file_repository : give the posibility to upload or download file from an sftp, ftp, filesystem repository

4: abstract_task : is needed for file_document. It just add the concept of task and this task can have several cron (if you want to execute task at two specific hours of the day)

In order to avoid discution between two PR and as this work is in some way related to #1 I propose to discuse their about the architecture of the whole system