Refactor Project Structure

[evanlouie]

• All packages except the top level cmd/cli/main.go are now internal
  • Note that internal directories has a special meaning in golang.
• To build, we now run go build -o fab cmd/cli/main.go
• Refactored Helm functionality out of the helm generator into its own package (/internal/helm)
• Refactored Git functionality out of core into its own package (/internal/git)

closes #318