How do I call unique on a dask DataFrame ?

I get the following error if I try to call it the same way as for a regular pandas dataframe:

```
In [27]: len(np.unique(ddf[['col1','col2']].values))

AttributeError                            Traceback (most recent call last)
<ipython-input-27-34c0d3097aab> in <module>()
----> 1 len(np.unique(ddf[['col1','col2']].values))

/dir/anaconda2/lib/python2.7/site-packages/dask/dataframe/core.pyc in __getattr__(self, key)
1924             return self._constructor_sliced(merge(self.dask, dsk), name,
1925                                             meta, self.divisions)
-> 1926         raise AttributeError("'DataFrame' object has no attribute %r" % key)
1927
1928     def __dir__(self):

AttributeError: 'DataFrame' object has no attribute 'values'

```