Series.shift() doesn't work for categorical type
Not sure if this is intentional, but Series.shift() won't run with categorical dtypes:

``` python
ser = pd.Series(['a', 'b', 'c', 'd'], dtype="category")
ser.shift(1)
Traceback (most recent call last):

  File "<ipython-input-15-1a7536b0af06>", line 1, in <module>
    ser.shift(1)

  File "/.../pandas/core/generic.py", line 3394, in shift
    new_data = self._data.shift(periods=periods, axis=block_axis)

  File "/.../pandas/core/internals.py", line 2533, in shift
    return self.apply('shift', **kwargs)

  File "/.../pandas/core/internals.py", line 2497, in apply
    applied = getattr(b, f)(**kwargs)

  File "/.../pandas/core/internals.py", line 893, in shift
    new_values, fill_value = com._maybe_upcast(self.values)

  File "/.../pandas/core/common.py", line 1218, in _maybe_upcast
    new_dtype, fill_value = _maybe_promote(dtype, fill_value)

  File "/.../pandas/core/common.py", line 1124, in _maybe_promote
    if issubclass(np.dtype(dtype).type, compat.string_types):

TypeError: data type not understood

```
