store: Add IsKeyDir

IsKeyDir is needed so we can check if a given key is a directory. This
should be a public method because we need to check whether a request
includes Dir=true in etcdserver.

Addressed issue etcd-io#518 on github.

store/store.go

@@ -41,6 +41,7 @@ func init() {
 type Store interface {
 	Version() int
 	Index() uint64
+	IsKeyDir(nodePath string) bool
 
 	Get(nodePath string, recursive, sorted bool) (*Event, error)
 	Set(nodePath string, dir bool, value string, expireTime time.Time) (*Event, error)
@@ -100,6 +101,17 @@ func (s *store) Index() uint64 {
 	return s.CurrentIndex
 }
 
+func (s *store) IsKeyDir(nodePath string) bool {
+	node, _ := s.internalGet(nodePath)
+	// If there is an error, then either the path does not exist or it is not
+	// a dir. Either way, it is not a dir, so return false
+	if node != nil {
+		return node.IsDir()
+	} else {
+		return false
+	}
+}
+
 // Get returns a get event.
 // If recursive is true, it will return all the content under the node path.
 // If sorted is true, it will sort the content by keys.