add Mkdir

Fixes #11

Seems to work having a mode issue:
```
--- FAIL: TestMkdir (0.01 seconds)
	all_test.go:487: Mode drwxr-x---, expected -rwxrwxrwx
	all_test.go:236: (end of all tests; shutting down world)
FAIL
FAIL	_/usr/local/google/home/proppy/docker/vfuse	1.264s
```