Make db name-literal agnostic

Move the persistence characteristic of the store into the graph registry
and provide an API hook to get that information.

Add error return for init on a non-persistent store.

Updates #35.

cayley.go

@@ -134,6 +134,6 @@ func main() {
 		flag.Usage()
 	}
 	if err != nil {
-		glog.Fatalln(err)
+		glog.Errorln(err)
 	}
 }