Negative indexing into Series in 0.8.0
Something changed between 0.5.0 and 0.8.0 that has disallowed using negative indexes as you would into a sequence. For example:

In [3]: ser = pandas.Series( ['A','B','C'] )

```
In [4]: ser
Out[4]:
0    A
1    B
2    C

In [5]: ser[1]
Out[5]: 'B'

In [6]: ser[-1]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-6-3cbe0b873a9e> in <module>()
----> 1 ser[-1]

c:\python27\lib\site-packages\pandas\core\series.pyc in __getitem__(self, key)
    427     def __getitem__(self, key):
    428         try:
--> 429             return self.index.get_value(self, key)
    430         except InvalidIndexError:
    431             pass

c:\python27\lib\site-packages\pandas\core\index.pyc in get_value(self, series, k
ey)
    639         """
    640         try:
--> 641             return self._engine.get_value(series, key)
    642         except KeyError, e1:
    643             if len(self) > 0 and self.inferred_type == 'integer':

c:\python27\lib\site-packages\pandas\lib.pyd in pandas.lib.IndexEngine.get_value
 (pandas\src\tseries.c:104257)()

c:\python27\lib\site-packages\pandas\lib.pyd in pandas.lib.IndexEngine.get_value
 (pandas\src\tseries.c:104085)()

c:\python27\lib\site-packages\pandas\lib.pyd in pandas.lib.IndexEngine.get_loc (
pandas\src\tseries.c:104794)()

c:\python27\lib\site-packages\pandas\lib.pyd in pandas.lib.Int64HashTable.get_it
em (pandas\src\tseries.c:15561)()

c:\python27\lib\site-packages\pandas\lib.pyd in pandas.lib.Int64HashTable.get_it
em (pandas\src\tseries.c:15515)()
```
