reindex raises AssertError after prior reindex
```
In [1]: import pandas

In [2]: import numpy as np

In [3]: index = pandas.date_range('1/1/2000', periods=8,)

In [4]: p = pandas.Panel(np.random.randn(2, len(index), 4), items=['Item1', 'Item2'], 
major_axis=index, minor_axis=['A', 'B', 'C', 'D'])

In [5]: p
Out[5]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 2 (items) x 8 (major) x 4 (minor)
Items: Item1 to Item2
Major axis: 2000-01-01 00:00:00 to 2000-01-08 00:00:00
Minor axis: A to D

In [7]: x = p.reindex(items = ['Item1','Item2'], copy = False)

In [8]: x.reindex(minor = ['A','B'])
Out[8]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 2 (items) x 8 (major) x 2 (minor)
Items: Item1 to Item2
Major axis: 2000-01-01 00:00:00 to 2000-01-08 00:00:00
Minor axis: A to B
```

This fails on a comparison in the internals.Block integrity_check of:
block.ref_items is self.items
they are in fact equal (e.g. ==), but id(block.ref_items) != id(self.items) and hence the is fails
since I first reindex on an index which is considered equal, you just return the index directly (rather
than create a new one as in an reindex where its not equal), then on the second reindex the block
manager thinks items has changed (to what the block sees), but its actually the other index  

I guess you could copy on an equals index, but not entirely sure this will solve the problem  

This is an odd and probably pathological case where I pre-reindexed some panels (on items), then reindexed another
axis later - passing copy = False on the reindex where there is the potential for an indentical index being passed is a work-around

```
In [10]: x = p.reindex(items = ['Item1','Item2'])

In [11]: x.reindex(minor = ['A','B'])
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-11-5f7bfc09b93d> in <module>()
----> 1 x.reindex(minor = ['A','B'])

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/panel.pyc in reindex(self, major, items, minor, method, major_axis, minor_axis, copy)
    691 
    692         if minor is not None:
--> 693             result = result._reindex_axis(minor, method, 2, copy)
    694 
    695         if items is not None:

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/generic.pyc in _reindex_axis(self, new_index, fill_method, axis, copy)
    589     def _reindex_axis(self, new_index, fill_method, axis, copy):
    590         new_data = self._data.reindex_axis(new_index, axis=axis,
--> 591                                            method=fill_method, copy=copy)
    592 
    593         if new_data is self._data and not copy:

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in reindex_axis(self, new_axis, method, axis, copy)
    959 
    960         new_axis, indexer = cur_axis.reindex(new_axis, method)
--> 961         return self.reindex_indexer(new_axis, indexer, axis=axis)
    962 
    963     def reindex_indexer(self, new_axis, indexer, axis=1, fill_value=np.nan):

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in reindex_indexer(self, new_axis, indexer, axis, fill_value)
    981         new_axes = list(self.axes)
    982         new_axes[axis] = new_axis
--> 983         return BlockManager(new_blocks, new_axes)
    984 
    985     def _reindex_indexer_items(self, new_items, indexer, fill_value):

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in __init__(self, blocks, axes, do_integrity_check)
    491 

/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/internals.pyc in _verify_integrity(self)
    572         mgr_shape = self.shape
    573         for block in self.blocks:
--> 574             assert(block.ref_items is self.items)
    575             assert(block.values.shape[1:] == mgr_shape[1:])
    576         tot_items = sum(len(x.items) for x in self.blocks)

AssertionError: 

```
