Access DateTimeIndexed dataframe by timestamp
Hello, 

I am new to pandas and thanks for this great library!

I have a data frame like this: 

```
Gold_2012.head()

                              open  high    low close   volume
date_time                   
2012-01-02 18:01:00  1571.0  1571.0  1569.1  1569.8  351
2012-01-02 18:02:00  1569.8  1570.0  1569.7  1569.8  54
2012-01-02 18:03:00  1570.0  1570.0  1569.1  1569.9  247
2012-01-02 18:04:00  1570.0  1570.0  1569.8  1569.9  55
2012-01-02 18:05:00  1569.8  1569.9  1568.5  1568.5  48
```

I am trying to access the first element of this dataframe. If I use loc function, everything works out:

```
Gold_2012.loc[Gold_2012.index[0]]


open      1571.0
high      1571.0
low       1569.1
close     1569.8
volume     351.0
Name: 2012-01-02 18:01:00-06:00, dtype: float64
```

But if I do something like this, an error is thrown. Is this expected?

```
Gold_2012[Gold_2012.index[0]]
```

---

KeyError                                  Traceback (most recent call last)
<ipython-input-30-bb7117766fdd> in <module>()
----> 1 Gold_2012[Gold_2012.index[0]]

/Users/chen/Virtualenvs/python3Env/lib/python3.3/site-packages/pandas/core/frame.py in **getitem**(self, key)
   1926         else:
   1927             # get column
-> 1928             return self._get_item_cache(key)
   1929 
   1930     def _getitem_slice(self, key):

/Users/chen/Virtualenvs/python3Env/lib/python3.3/site-packages/pandas/core/generic.py in _get_item_cache(self, item)
    568             return cache[item]
    569         except Exception:
--> 570             values = self._data.get(item)
    571             res = self._box_item_values(item, values)
    572             cache[item] = res

/Users/chen/Virtualenvs/python3Env/lib/python3.3/site-packages/pandas/core/internals.py in get(self, item)
   1382 
   1383     def get(self, item):
-> 1384         _, block = self._find_block(item)
   1385         return block.get(item)
   1386 

/Users/chen/Virtualenvs/python3Env/lib/python3.3/site-packages/pandas/core/internals.py in _find_block(self, item)
   1524 
   1525     def _find_block(self, item):
-> 1526         self._check_have(item)
   1527         for i, block in enumerate(self.blocks):
   1528             if item in block:

/Users/chen/Virtualenvs/python3Env/lib/python3.3/site-packages/pandas/core/internals.py in _check_have(self, item)
   1531     def _check_have(self, item):
   1532         if item not in self.items:
-> 1533             raise KeyError('no item named %s' % com.pprint_thing(item))
   1534 
   1535     def reindex_axis(self, new_axis, method=None, axis=0, copy=True):

KeyError: 'no item named 2012-01-02 18:01:00-06:00'
