BUG: groupby.agg with more than one lambda is not allowed?
consider the following example

```
 a = rand(100)
 b = np.floor(rand(100)*100)

 df = pd.DataFrame({'a' : a , 'b' : b})

 grp = df.groupby(df.b)    
```

I have grouped the values in `a` by `b`. 

Now, if I want to plot the trend over the groups with mean and std I can do

```
grp.a.agg([np.mean, lambda x : np.mean(x) + np.std(x) , lambda x :  np.mean(x) - np.std(x) ]).plot()
```

which gives me

```
SpecificationError: Function names must be unique, found multiple named <lambda>
```

while 

```
  grp.a.agg([np.mean, lambda x : np.mean(x) + np.std(x) ]).plot()
```

which has just one lambda works ok. 

Is this a bug?

In order to make the thing work I had to define real functions (i.e. in terms of `def`), to be put in `agg`.
