CLI tool with multiple commands to list and sync a bucket.

Following my first PR, I've reworked the code to fit in a unique binary that has commands.

* `brigade list [options]`: list a bucket into a json.gz file.
* `brigade slice [options]`: slice a bucket list into sub lists, all in json.gz form.
* `brigade diff [options]`: compare an old bucket listing with a new one, looking at ETags that are new.
* `brigade sync [options]`: sync two buckets given a file listing.

Each command is tested.  Since the discussion as to how we want to sync is still ongoing, the `sync` command has pluggable sync method.