Rework backend interface for easier integration

[tiesselune]

Backend interface is clumsy.

The core code should handle push/pull advancement status updates and display, while backends should push and pull just one object at a time.

This would reduce code copy/paste and allow more controll over textual feedback in the console, along with far easier backend programming.

[tiesselune]

As mentionned in issue #10, It would be great to have a piping system before pushing (or after pulling) to do some tasks like compressing/decompressing, and have a simple interface for those as well so that anybody can extend the system.