ENH: Add .topK(col, k) to DataFrame
When exploring new datasets in pandas, I often find myself writing

`````` df.groupby(col)[col].agg(len).order(ascending=False).head()```.

I added a convenience method to frame.py so that I could write ```df.topK(col)``` instead:

```python
def topK(self, col, k=5):
    """Returns top k most frequently occurring values in column col                                                                                                                                                                    
    """
    return self.groupby(col)[col].agg(len).order(ascending=False).head(k)
``````
