Squeeze fails on zero length arrays
``` python
In [77]: empty=pd.DataFrame(pd.np.empty(shape=(0,1)))

In [78]: empty
Out[78]: 
Empty DataFrame
Columns: [0]
Index: []

In [79]: empty.squeeze()
Out[79]: 
Empty DataFrame
Columns: [0]
Index: []
```

...but should return a `Series`

PR coming now
