set default store, fix store options bug, add String method

defaults.go

@@ -3,10 +3,12 @@ package micro
 import (
 	"github.com/micro/go-micro/client"
 	"github.com/micro/go-micro/server"
+	"github.com/micro/go-micro/store"
 
 	// set defaults
 	gcli "github.com/micro/go-micro/client/grpc"
 	gsrv "github.com/micro/go-micro/server/grpc"
+	memStore "github.com/micro/go-micro/store/memory"
 )
 
 func init() {
@@ -14,4 +16,6 @@ func init() {
 	client.DefaultClient = gcli.NewClient()
 	// default server
 	server.DefaultServer = gsrv.NewServer()
+	// default store
+	store.DefaultStore = memStore.NewStore()
 }