groupby enumerate method
I'm not sure what a good word for this is (count is taken, order means sort)!

But it's quite an often used thing to create a column which enumerates the items in each group / counts their occurrences.

You can hack it:

```
In [1]: df = pd.DataFrame([[1, 2], [2, 3], [1, 4], [1, 5], [2, 6]])

In [2]: g = df.groupby(0)

In [3]: g.apply(lambda x: pd.Series(np.arange(len(x)), x.index))
Out[3]:
0    0
1    0
2    1
3    2
4    1
dtype: int64

In [5]: df['order'] = _

In [6]: df
Out[6]:
   0  1  order
0  1  2      0
1  2  3      0
2  1  4      1
3  1  5      2
4  2  6      1
```

_I've seen this in a few SO questions, here's just [one](http://stackoverflow.com/questions/18392283/pandas-return-first-item-in-dataframe-grouped-by-user)._

cc @cpcloud (and I've seen @jreback answer a question with this)
