slicing with dt object or pd.Timestamp on unsorted date index fails
#### Code Sample, a copy-pastable example if possible

```python
data = list(zip([dt.date(2011, 1, 1), dt.date(2007, 1, 1), dt.date(2015, 1, 1)], [1, 1, 1]))
df = pd.DataFrame(data, columns=['date', 'value']).set_index('date')
df.index = pd.to_datetime(df.index)

df['2005':]  # works
df[dt.date(2005, 1, 1):] # fail
df[dt.datetime(2005, 1, 1):] # fail
df[dt.datetime(2005, 1, 1, 0, 0):] # fail
df[pd.Timestamp('20050101'):] # fail

df = df.sort_index()
df['2005':]  # works
df[dt.date(2005, 1, 1):] # works
df[dt.datetime(2005, 1, 1):] # works
df[dt.datetime(2005, 1, 1, 0, 0):] # works
df[pd.Timestamp('20050101'):] # works


```
#### Problem description

I am not sure why you could slice by string representation of a date but no the date object. when index is sorted, all call works.

#### Expected Output
same as sorted
#### Output of ``pd.show_versions()``
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None

<details>

[paste the output of ``pd.show_versions()`` here below this line]

</details>
