BUG: SparsePanel cross sectional not working
My understanding is that SparsePanel is supposed to have the same API as Panel, so I expected `sparse_pan[:,:,1]` to work:

``` python
In [11]: data = np.zeros((3,3,3))

In [12]: data[:] = np.nan

In [13]: data[1,1,1] = 1

In [14]: pan = pd.Panel(data)

In [15]: pan[:,:,1]
Out[15]: 
    0   1   2
0 NaN NaN NaN
1 NaN   1 NaN
2 NaN NaN NaN

In [16]: sparse_pan = pan.to_sparse()

In [17]: sparse_pan[:,:,1]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-17-e57ddf7a7092> in <module>()
----> 1 sparse_pan[:,:,1]

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/panel.py in __getitem__(self, key)
    256         if lib.isscalar(key):
    257             return super(Panel, self).__getitem__(key)
--> 258         return self.ix[key]
    259 
    260     def _getitem_multilevel(self, key):

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/indexing.py in __getitem__(self, key)
     68                 pass
     69 
---> 70             return self._getitem_tuple(key)
     71         else:
     72             return self._getitem_axis(key, axis=0)

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/indexing.py in _getitem_tuple(self, tup)
    697     def _getitem_tuple(self, tup):
    698         try:
--> 699             return self._getitem_lowerdim(tup)
    700         except IndexingError:
    701             pass

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/indexing.py in _getitem_lowerdim(self, tup)
    822         for i, key in enumerate(tup):
    823             if _is_label_like(key) or isinstance(key, tuple):
--> 824                 section = self._getitem_axis(key, axis=i)
    825 
    826                 # we have yielded a scalar ?

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/indexing.py in _getitem_axis(self, key, axis)
    926                     return self._get_loc(key, axis=axis)
    927 
--> 928             return self._get_label(key, axis=axis)
    929 
    930     def _getitem_iterable(self, key, axis=0):

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/indexing.py in _get_label(self, label, axis)
     86             raise IndexingError('no slices here, handle elsewhere')
     87 
---> 88         return self.obj._xs(label, axis=axis)
     89 
     90     def _get_loc(self, key, axis=0):

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/panel.py in xs(self, key, axis, copy)
    787         self._consolidate_inplace()
    788         axis_number = self._get_axis_number(axis)
--> 789         new_data = self._data.xs(key, axis=axis_number, copy=False)
    790         result = self._construct_return_type(new_data)
    791         copy = new_data.is_mixed_type

/home/will/.virtualenvs/dev3/lib/python3.4/site-packages/pandas/core/generic.py in __getattr__(self, name)
   1938 
   1939         if name in self._internal_names_set:
-> 1940             return object.__getattribute__(self, name)
   1941         elif name in self._metadata:
   1942             return object.__getattribute__(self, name)

AttributeError: 'SparsePanel' object has no attribute '_data'
```

No surprise: same behavior with ix and iloc.

Accessing via `minor_xs` does work

```
In [23]: sparse_pan.minor_xs(1)
Out[23]: 
    0   1   2
0 NaN NaN NaN
1 NaN   1 NaN
2 NaN NaN NaN
```

``` python
In [3]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.1.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-431.17.1.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.15.2
nose: 1.3.4
Cython: 0.20.2
numpy: 1.9.1
scipy: 0.14.1
```
