groupby string function-name shortcuts don't work with transform
```
df.groupby('key1')['data1'].transform('mean')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/book_scripts/notebooks/<ipython-input-42-46fdb55e8ac6> in <module>()
----> 1 df.groupby('key1')['data1'].transform('mean')

/home/wesm/code/pandas/pandas/core/groupby.pyc in transform(self, func, *args, **kwargs)
   1298         for name, group in self:
   1299             object.__setattr__(group, 'name', name)
-> 1300             res = func(group, *args, **kwargs)
   1301             indexer = self.obj.index.get_indexer(group.index)
   1302             np.put(result, indexer, res)

TypeError: 'str' object is not callable
```
