BUG: partial slicing with a Timestamp on PeriodIndex
An example: a PeriodicIndex with a freq of 300S. First second works.
Remaining portion of the interval raises a KeyError.
```
>>> DR = pd.period_range(datetime.datetime.now(), freq='300S',periods=22)
>>> S = pd.Series( [0.0]*22,index=DR) 
>>> now = datetime.datetime.now()
>>> S[now]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/pandas/core/series.py", line 603, in __getitem__
    result = self.index.get_value(self, key)
  File "/Library/Python/2.7/site-packages/pandas/tseries/period.py", line 757, in get_value
    return com._maybe_box(self, self._engine.get_value(s, key),
  File "pandas/index.pyx", line 98, in pandas.index.IndexEngine.get_value (pandas/index.c:3557)
  File "pandas/index.pyx", line 106, in pandas.index.IndexEngine.get_value (pandas/index.c:3240)
  File "pandas/index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas/index.c:4279)
  File "pandas/src/hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8564)
  File "pandas/src/hashtable_class_helper.pxi", line 410, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8508)
KeyError: 1491422454
>>> S.index[0]
Period('2017-04-05 20:00:23', '300S')
>>> S[datetime.datetime(2017,4,5,20,0,23)]
0.0
>>> S[datetime.datetime(2017,4,5,20,0,23,999)]
0.0
```

```
>>> DR = pd.period_range(datetime.datetime.now(), freq='5T',periods=22)
>>> S = pd.Series( [0.0]*22,index=DR)
>>> now = datetime.datetime.now()
>>> S[now]
0.0
>>> S.index[0]
Period('2017-04-05 20:03', '5T')
>>> S[datetime.datetime(2017,4,5,20,3,23,999)]
0.0
>>> S[datetime.datetime(2017,4,5,20,4,23,999)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Python/2.7/site-packages/pandas/core/series.py", line 603, in __getitem__
    result = self.index.get_value(self, key)
  File "/Library/Python/2.7/site-packages/pandas/tseries/period.py", line 757, in get_value
    return com._maybe_box(self, self._engine.get_value(s, key),
  File "pandas/index.pyx", line 98, in pandas.index.IndexEngine.get_value (pandas/index.c:3557)
  File "pandas/index.pyx", line 106, in pandas.index.IndexEngine.get_value (pandas/index.c:3240)
  File "pandas/index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas/index.c:4279)
  File "pandas/src/hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8564)
  File "pandas/src/hashtable_class_helper.pxi", line 410, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8508)
KeyError: 24857044
```
<details>
pandas.version
u'0.19.2'
</details>