Missing Series name when using count() on groupby object
Consider this very basic group by operation:

``` python
df = pd.DataFrame({'col1': ['a', 'b', 'a', 'c'], 'col2': [1, 3, 2, 5]})
df_grp = df.groupby('col1')['col2']
```

When aggregating using `mean`, `min`, ... (everything but `count`) the resulting Series has `col2` as its name:

``` python
df_grp.mean()

col1
a       1.5
b       3.0
c       5.0
Name: col2, dtype: float64
```

However, if I use `count` for aggregation, the name is not set:

``` python
df_grp.count()

col1
a       2
b       1
c       1
dtype: int64
```

Not a big problem for a simple case like that, but I stumbled over that while working with a MultiIndex that needed to be reindexed and led to a `KeyError` due to that missing column name.
