pandas.floor('H') does not work with DST
Using floor on a DatetimeIndex that has a timezone with DST fails when converting the result back to the tz as there may be ambiguous time (see example below).

``` python
# create 15' date range with Brussels tz
idx = pandas.date_range("2016-01-01","2017-01-01",freq="15T", tz="Europe/Brussels")
# transform the index to get rid of minutes
idx_h = idx.floor("H")
```
#### Expected Output

idx_h rounded to the hour
#### Actual Output

excepection raised when relocalizing the index after applying the rouding

``` python
Traceback (most recent call last):
  File "...", line 118, in <module>
    idx_h = idx.floor("H")
  File "...\lib\site-packages\pandas\tseries\base.py", line 98, in floor
    return self._round(freq, np.floor)
  File "...\lib\site-packages\pandas\tseries\base.py", line 89, in _round
    result = result.tz_localize(self.tz)#, ambiguous=[d.dst() for d in self])
  File "...\lib\site-packages\pandas\util\decorators.py", line 91, in wrapper
    return func(*args, **kwargs)
  File "...\lib\site-packages\pandas\tseries\index.py", line 1857, in tz_localize
    ambiguous=ambiguous)
  File "pandas\tslib.pyx", line 4087, in pandas.tslib.tz_localize_to_utc (pandas\tslib.c:69556)
pytz.exceptions.AmbiguousTimeError: Cannot infer dst time from Timestamp('2016-10-30 02:00:00'), try using the 'ambiguous' argument
```
#### Possible solution

The code to reconvert to local tz need to have 'ambiguous' defined and can use the original array of dst flags.
Lines 87-89 in ...\Lib\site-packages\pandas\tseries\base.py

``` python
        # reconvert to local tz
        if getattr(self, 'tz', None) is not None:
            result = result.tz_localize(self.tz)
```

to be replaced by
Lines 87-89 in ...\Lib\site-packages\pandas\tseries\base.py

``` python
        # reconvert to local tz
        if getattr(self, 'tz', None) is not None:
            result = result.tz_localize(self.tz, ambiguous=[d.dst() for d in self])
```

Would it be also useful to have a DatetimeIndex.dst function to return a boolean array reuseable in tz_localize ?
#### Output of `pd.show_versions()`

<details>
# Paste the output here
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.0
nose: 1.3.7
pip: 8.1.1
setuptools: 20.3
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
</details>
