Initial work with guitar.
Merge branch 'master' into guitar

Conflicts:
	commands/build.go
Don't crash when exiting normally from 'git show'
Add upstream & index fields to config, clear default image name
Use upstream & index fields in build command
Commit image as upstream if sourced from index
work on improving bootstrap of a git graph repo
Merge branch 'smooth-git-bootstrap' into guitar
Don't run git show
Updated quickstart & build instructions
Commit the image to docket unconditionally.
Correctly detect graph status if graph is a symlink. Fixes #15.
Add stack-trace handling to main function. Fixes #14.
Fixed comparison error. Fixes #13.
Do not try to load from graph if image branch is not found. Fixes #12.
Add flags to git clean. Fixes #16.