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

app.go

@@ -242,6 +242,10 @@ func (a *App) Run(arguments []string) (err error) {
 		}
 	}
 
+	if a.Action == nil {
+		a.Action = helpCommand.Action
+	}
+
 	// Run default Action
 	err = HandleAction(a.Action, context)