groupby problem when dataframe has only one column
```
>>> import pandas as pd 
>>> import numpy as np
>>> df = pd.DataFrame(index=range(10),data=np.random.randint(0,3,10),columns=['y'])
```

Gives,

```
>>> df.y
0    2
1    0
2    2
3    1
4    0
5    0
6    0
7    1
8    2
9    1
Name: y, dtype: int32

>>> df.groupby('y').count()
Empty DataFrame
Columns: []
Index: [0, 1, 2]
```

But I'm trying to get the count of every item in 'y'. Something like,

```
>>> from collections import Counter
>>> Counter(df.y)
Counter({0: 4, 1: 3, 2: 3})
```

Am I doing something wrong here?

Thanks!
