Allow autocompletion of git branches for all Git commands

This is safer. The old way prevented autocompletion for normal
Git commands.
Improved features for git-town
resolves #199: command list architecture
Add missing feature specs for git ship
Missing feature specs for git kill
Improved features for git-extract
Improved features for git-prune-branches
Improved features for git-kill
resolve #271: git-pr/git-repo: make .git optional
resolve #269: update git-extract errors
resolve #270: catch shell errors
resolve #196: git kill: deleted tracking branch
Update release-notes.md
Improved features for git-ship
Merge branch 'master' into kg-user-stories-hack