concat DataFrames bug with keys on axis=1
```

In [11]: t1
Out[11]: 
    value
id       
a   1    
b   2    
c   3    

In [12]: t2
Out[12]: 
    value
id       
a   7    
b   8    

In [13]: concat([t1, t2], axis=1, keys=['t1', 't2'])
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/<ipython-input-13-8c00deb927ee> in <module>()
----> 1 concat([t1, t2], axis=1, keys=['t1', 't2'])

/home/wesm/code/pandas/pandas/tools/merge.pyc in concat(objs, axis, join, join_axes, ignore_index, keys, levels, names, verify_integrity)
    649                        keys=keys, levels=levels, names=names,
    650                        verify_integrity=verify_integrity)
--> 651     return op.get_result()
    652 
    653 

/home/wesm/code/pandas/pandas/tools/merge.pyc in get_result(self)
    715             return Series(new_data, index=self.new_axes[0], name=name)
    716         else:
--> 717             new_data = self._get_concatenated_data()
    718             return self.objs[0]._from_axes(new_data, self.new_axes)
    719 

/home/wesm/code/pandas/pandas/tools/merge.pyc in _get_concatenated_data(self)
    735                 stacked_block = self._concat_blocks(klass_blocks)
    736                 new_blocks.append(stacked_block)
--> 737             new_data = BlockManager(new_blocks, self.new_axes)
    738         except Exception:  # EAFP
    739             # should not be possible to fail here for the expected reason with


/home/wesm/code/pandas/pandas/core/internals.pyc in __init__(self, blocks, axes, do_integrity_check)
    283 
    284         if do_integrity_check:
--> 285             self._verify_integrity()
    286 
    287     def __nonzero__(self):

/home/wesm/code/pandas/pandas/core/internals.pyc in _verify_integrity(self)
    362 
    363     def _verify_integrity(self):
--> 364         _union_block_items(self.blocks)
    365         mgr_shape = self.shape
    366         for block in self.blocks:

/home/wesm/code/pandas/pandas/core/internals.pyc in _union_block_items(blocks)
   1071 
   1072     if tot_len > len(the_union):
-> 1073         raise Exception('item names overlap')
   1074     return the_union
   1075 

Exception: item names overlap
```
