Support ExtensionArrarys for shift

[TomAugspurger]

This is needed for sparse. I have a separate PR incoming

In [2]: import pandas as pd

In [3]: pd.Series([1, 2], dtype='Int8').shift()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-8c2f15affd64> in <module>()
----> 1 pd.Series([1, 2], dtype='Int8').shift()

~/sandbox/pandas/pandas/core/series.py in shift(self, periods, freq, axis)
   3476     @Appender(generic._shared_docs['shift'] % _shared_doc_kwargs)
   3477     def shift(self, periods=1, freq=None, axis=0):
-> 3478         return super(Series, self).shift(periods=periods, freq=freq, axis=axis)
   3479
   3480     def reindex_axis(self, labels, axis=0, **kwargs):

~/sandbox/pandas/pandas/core/generic.py in shift(self, periods, freq, axis)
   8088         block_axis = self._get_block_manager_axis(axis)
   8089         if freq is None:
-> 8090             new_data = self._data.shift(periods=periods, axis=block_axis)
   8091         else:
   8092             return self.tshift(periods, freq)

~/sandbox/pandas/pandas/core/internals/managers.py in shift(self, **kwargs)
    545
    546     def shift(self, **kwargs):
--> 547         return self.apply('shift', **kwargs)
    548
    549     def fillna(self, **kwargs):

~/sandbox/pandas/pandas/core/internals/managers.py in apply(self, f, axes, filter, do_integrity_check, consolidate, **kwargs)
    421
    422             kwargs['mgr'] = self
--> 423             applied = getattr(b, f)(**kwargs)
    424             result_blocks = _extend_blocks(applied, result_blocks)
    425

~/sandbox/pandas/pandas/core/internals/blocks.py in shift(self, periods, axis, mgr)
   1289
   1290         # make sure array sent to np.roll is c_contiguous
-> 1291         f_ordered = new_values.flags.f_contiguous
   1292         if f_ordered:
   1293             new_values = new_values.T

AttributeError: 'IntegerArray' object has no attribute 'flags'