Run script at top level of git repo

This fixes two issues: (1) running the script **outside** a git repo,
and (2) running the script in a subdirectory of a git repo, instead of
running it at the top level of the repo
Merge pull request #7 from pvdb/run_from_actual_git_dir

Run "git branch-delete-orphans" at top level of git repo
Bump to 0.1.0
Update changelog
Pimp CHANGELOG
Add rake to gemspec