Start MemMapFS with a root directory.

memmap.go

diff --git a/memmap.go b/memmap.go
index 3bf85f7..efd0db7 100644
--- a/memmap.go
+++ b/memmap.go
@@ -44,6 +44,10 @@
 func (m *MemMapFs) getData() map[string]File {
 	if m.data == nil {
 		m.data = make(map[string]File)
+
+		// Root should always exist, right?
+		// TODO: what about windows?
+		m.getData()["/"] = &InMemoryFile{name: "/", memDir: &MemDirMap{}, dir: true}
 	}
 	return m.data
 }