Fallback to either nano, pico, or vi if no editor is set.

This fixes #119.
Deduplicate and sort targets in the generated message title.
Update change log.
Update tests, now that items in the message title are sorted.

Also, moved CommitMessage inline with Pinto::Role::Committable
because I just didn't like having a separate class that looks
out of place.
Merge branch 'master' into feature/recursion-defaults

Conflicts:
	Changes