Incremental backup

[wachterjohannes]

Backup should be able to define "previous" backup. The backup command will have a new options --incremental. In the archive only new file will be saved and a list of deleted files.

This has to be handled in:

1. Restore: A ChainAdapter which receives a list of adapters in the right order and go thruh them foreach function in AdapterInterface.
2. Fetch & Get: load also the dependencies
3. Information: should display all the dependencies

@chirimoya @c00n84 can you add some thoughts?