KeyError for crosstab on Series with same name.
Doing a crosstab on two Series with the same name throws an error.  This is due to a dictionary (indexed by the series name) in the crosstab function being used to store the data.  Not sure if this is a feature or a bug, but a default similar to the behavior when Series without name are compared would be desirable to me. 

``` python
In [56]:

s1 = pd.Series([1,1,2,2,3,3], name='s')
s2 = pd.Series([1,1,1,2,2,2], name='s')

pd.crosstab(s1, s2)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-56-9d16b2abac9f> in <module>()
      2 s2 = pd.Series([1,1,1,2,2,2], name='s')
      3 
----> 4 pd.crosstab(s1, s2)

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/tools/pivot.pyc in crosstab(rows, cols, values, rownames, colnames, aggfunc, margins, dropna)
    368         df['__dummy__'] = 0
    369         table = df.pivot_table('__dummy__', rows=rownames, cols=colnames,
--> 370                                aggfunc=len, margins=margins, dropna=dropna)
    371         return table.fillna(0).astype(np.int64)
    372     else:

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/tools/pivot.pyc in pivot_table(data, values, rows, cols, aggfunc, fill_value, margins, dropna)
    108         to_unstack = [agged.index.names[i]
    109                       for i in range(len(rows), len(keys))]
--> 110         table = agged.unstack(to_unstack)
    111 
    112     if not dropna:

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/core/frame.pyc in unstack(self, level)
   3339         """
   3340         from pandas.core.reshape import unstack
-> 3341         return unstack(self, level)
   3342 
   3343     #----------------------------------------------------------------------

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/core/reshape.pyc in unstack(obj, level)
    416 def unstack(obj, level):
    417     if isinstance(level, (tuple, list)):
--> 418         return _unstack_multiple(obj, level)
    419 
    420     if isinstance(obj, DataFrame):

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/core/reshape.pyc in _unstack_multiple(data, clocs)
    275     index = data.index
    276 
--> 277     clocs = [index._get_level_number(i) for i in clocs]
    278 
    279     rlocs = [i for i in range(index.nlevels) if i not in clocs]

/cellar/users/agross/anaconda2/lib/python2.7/site-packages/pandas-0.13.0_247_g82bcbb8-py2.7-linux-x86_64.egg/pandas/core/index.pyc in _get_level_number(self, level)
   2197         except ValueError:
   2198             if not isinstance(level, int):
-> 2199                 raise KeyError('Level %s not found' % str(level))
   2200             elif level < 0:
   2201                 level += self.nlevels

KeyError: 'Level s not found'
```
