Remove target with delegations

[jku]

This started with just refactoring the remove-target (so it matches add-target) but turned into a collection of multiple changes:

• make remove-target like add-target (it is a top level command and follows delegations too)
• refactor both remove-target and add-target so that the generic parts are in Repository and only git-related parts are in GitRepository
• make timestamp a top-level command as well
• separate timestamp functionality out of *Repository.snapshot() into it's own method (making sure the obsolete snapshot files are always deleted when they become obsolete)
• remove workaround in edit() where some files would be deleted there: now only GitRepository.timestamp() and GitRepository.snapshot() delete files (because only they make files obsolete)
• Improve README

Should be noted that while timestamp and snapshot are now separated in the Repository class, the CLI snapshot command still does a timestamp if needed.

Fixes #46, fixes #47, fixes #31

[vmwclabot]

@jku, VMware has approved your signed contributor license agreement.

[jku]

I'll merge this so #48 can progress: feel free to open new issues for review comments