Negative indexes does not work as in Python/Numpy (as one would expect)
Could not find a reference for this anywhere, so I'm sorry if already covered in the docs or in another issue.

```
$ python
Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = [1,2,3,4,5]
>>> a[-1]
5
>>> import numpy
>>> npa = numpy.array([1,2,3,4,5])
>>> npa[-1]
5
>>> import pandas
>>> s = pandas.Series([1,2,3,4,5])
>>> s[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.6/dist-packages/pandas-0.7.2-py2.6-linux-i686.egg/pandas/core/series.py", line 376, in __getitem__
    return self.index.get_value(self, key)
  File "/usr/local/lib/python2.6/dist-packages/pandas-0.7.2-py2.6-linux-i686.egg/pandas/core/index.py", line 516, in get_value
    return self._engine.get_value(series, key)
  File "engines.pyx", line 21, in pandas._engines.IndexEngine.get_value (pandas/src/engines.c:1558)
  File "engines.pyx", line 29, in pandas._engines.IndexEngine.get_value (pandas/src/engines.c:1437)
  File "engines.pyx", line 101, in pandas._engines.DictIndexEngine.get_loc (pandas/src/engines.c:2664)
  File "engines.pyx", line 108, in pandas._engines.DictIndexEngine.get_loc (pandas/src/engines.c:2616)
KeyError: -1
>>>
```

Notice that if indexes aren't numeric, everything work as expected:

```
>>> s = pandas.Series([1,2,3,4,5],index=['a','b','c','d','e'])
>>> s[-1]
5
```

Is this a bug or "by design"?
