Simplify code to run commands

provide better error messages when someone is missing git or elm-doc.
Cosmetic: mess with indentation
Add check for git
Bump version number
Get dependencies in line with recent shenanigans
Add instances of Show and FromJSON for Listing
Merge branch 'master' of https://github.com/evancz/elm-get
Remove unimplemented Update command from interface
Update copyright
Bump version
Merge branch 'master' into server_refactor

Conflicts:
	elm-get.cabal