TST: more testing of indexing with sparse
related is #6076 (`loc` not working either)

It would be nice if common operations like indexing/slicing is available for sparse dataframes as for their dense counterparts.

Thanks!

``` python
In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: A = np.zeros((3,4))

In [4]: A[:] = np.nan

In [5]: A[2,3] = 1

In [6]: 

In [6]: Y = pd.DataFrame(A)

In [7]: print Y.iloc[2,3]   # print 1.0
1.0

In [8]: sY = Y.to_sparse()

In [9]: print sY.iloc[2,3]
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-0674f68ef869> in <module>()
----> 1 print sY.iloc[2,3]

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/indexing.pyc in __getitem__(self, key)
    667     def __getitem__(self, key):
    668         if type(key) is tuple:
--> 669             return self._getitem_tuple(key)
    670         else:
    671             return self._getitem_axis(key, axis=0)

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/indexing.pyc in _getitem_tuple(self, tup)
    774                 continue
    775 
--> 776             retval = getattr(retval,self.name)._getitem_axis(key, axis=i)
    777 
    778         return retval

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/indexing.pyc in _getitem_axis(self, key, axis)
    803                 raise ValueError("Cannot index by location index with a non-integer key")
    804 
--> 805             return self._get_loc(key,axis=axis)
    806 
    807     def _convert_to_indexer(self, obj, axis=0):

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/indexing.pyc in _get_loc(self, key, axis)
     61 
     62     def _get_loc(self, key, axis=0):
---> 63         return self.obj._ixs(key, axis=axis)
     64 
     65     def _slice(self, obj, axis=0, raise_on_error=False):

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/frame.pyc in _ixs(self, i, axis, copy)
   1880                         new_values = self._data.fast_2d_xs(i, copy=copy)
   1881                     except:
-> 1882                         new_values = self._data.fast_2d_xs(i, copy=True)
   1883                     return Series(new_values, index=self.columns,
   1884                                   name=self.index[i])

AttributeError: '_SparseMockBlockManager' object has no attribute 'fast_2d_xs'
```
