Make `move_dir()` atomic again

Why it is not with it's current implementation?

```
❯ python
Python 2.6.6 (r266:84292, Aug  4 2014, 14:47:49) 
[GCC 4.4.5] on linux3
Type "help", "copyright", "credits" or "license" for more information.
>>> 
>>> import luigi.hdfs
luigi/hdfs.py:576: UserWarning: Deprecated: Specify 'client: hadoopcli' in config
  warnings.warn("Deprecated: Specify 'client: hadoopcli' in config")
>>> hd_client = luigi.hdfs.HdfsClient()
>>> 
>>> hd_client.mkdir('wohoo', parents=False, raise_if_exists=False)
>>> hd_client.mkdir('wohoo', parents=False, raise_if_exists=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "luigi/hdfs.py", line 238, in mkdir
    raise FileAlreadyExists(ex.stderr)
luigi.target.FileAlreadyExists: mkdir: `wohoo': File exists
```

It raises if it already exists. (I disabled the assertion to run the above code). 

---

But I suppose you mean why I don't change the implementation to allow that?