BUG: indexing error when setting item in empty Series which has a frequency
Another somewhat degenerate case that works in 0.15.2, fails in 0.16.1

``` python
import pandas as pd

pd.show_versions()


ts = pd.TimeSeries()

ts[pd.datetime(2012, 1, 1)] = 47


ts2 = pd.TimeSeries(0, pd.date_range('2011-01-01', '2011-01-01'))[:0]

ts2[pd.datetime(2012, 1, 1)] = 47
```

0.15.2 output:

``` python
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 26 Stepping 5, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.15.2
nose: 1.3.4
Cython: 0.21
numpy: 1.9.2
scipy: 0.14.0
statsmodels: 0.5.0
IPython: 3.0.0-dev
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.2
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.5.7
lxml: 3.4.0
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.7
pymysql: None
psycopg2: None
```

0.16.1 output:

``` python
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 26 Stepping 5, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.16.1-46-g0aceb38
nose: 1.3.6
Cython: 0.22
numpy: 1.9.2
scipy: 0.14.0
statsmodels: 0.6.1
IPython: 3.1.0
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.7.2
lxml: None
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.0.4
pymysql: None
psycopg2: None

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
c:\test_set_empty_series_with_freq.py in <module>()
     11 ts2 = pd.TimeSeries(0, pd.date_range('2011-01-01', '2011-01-01'))[:0]
     12
---> 13 ts2[pd.datetime(2012, 1, 1)] = 47

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\core\series.pyc in __setitem__(self, key, value)
    687         # do the setitem
    688         cacher_needs_updating = self._check_is_chained_assignment_possible()
--> 689         setitem(key, value)
    690         if cacher_needs_updating:
    691             self._maybe_update_cacher()

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\core\series.pyc in setitem(key, value)
    660                             pass
    661                 try:
--> 662                     self.loc[key] = value
    663                 except:
    664                     print ""

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\core\indexing.pyc in __setitem__(self, key, value)
    113     def __setitem__(self, key, value):
    114         indexer = self._get_setitem_indexer(key)
--> 115         self._setitem_with_indexer(indexer, value)
    116
    117     def _has_valid_type(self, k, axis):

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\core\indexing.pyc in _setitem_with_indexer(self, indexer, value)
    272                 if self.ndim == 1:
    273                     index = self.obj.index
--> 274                     new_index = index.insert(len(index),indexer)
    275
    276                     # this preserves dtype of the value

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\tseries\index.pyc in insert(self, loc, item)
   1523             # check freq can be preserved on edge cases
   1524             if self.freq is not None:
-> 1525                 if (loc == 0 or loc == -len(self)) and item + self.freq == self[0]:
   1526                     freq = self.freq
   1527                 elif (loc == len(self)) and item - self.freq == self[-1]:

c:\python\envs\pandas-0.16.1\lib\site-packages\pandas\tseries\index.pyc in __getitem__(self, key)
   1351         getitem = self._data.__getitem__
   1352         if np.isscalar(key):
-> 1353             val = getitem(key)
   1354             return Timestamp(val, offset=self.offset, tz=self.tz)
   1355         else:

IndexError: index 0 is out of bounds for axis 0 with size 0

```

Proposed https://github.com/pydata/pandas/pull/10194
