If no action is specified on the command or app, print the help documentation

Rather than panic'ing or displaying an opaque error message about the
signature which is more confusing to the end user.

Fixes #562

help.go

@@ -13,7 +13,7 @@ import (
 // cli.go uses text/template to render templates. You can
 // render custom help text by setting this variable.
 var AppHelpTemplate = `NAME:
-   {{.Name}} - {{.Usage}}
+   {{.Name}}{{if .Usage}} - {{.Usage}}{{end}}
 
 USAGE:
    {{if .UsageText}}{{.UsageText}}{{else}}{{.HelpName}} {{if .VisibleFlags}}[global options]{{end}}{{if .Commands}} command [command options]{{end}} {{if .ArgsUsage}}{{.ArgsUsage}}{{else}}[arguments...]{{end}}{{end}}{{if .Version}}{{if not .HideVersion}}