Not possible to find first element in non-monotonic DateTimeIndex if string is used
There is an inconsistency when you look for dates in a DateTimeIndex if you query using strings and the index is non-monotonic. The first element in the index cannot be found in this way, but if you manually convert it to a timestamp object, or if you look for elements other than the first, then the lookup is successful.
#### Code Sample, a copy-pastable example if possible

``` python
>>> import pandas as pd
>>> idx = pd.to_datetime(['2015-01-01', '2015-01-02', '2015-01-05', '2015-01-04'])
>>> idx
Out[38]: DatetimeIndex(['2015-01-01', '2015-01-02', '2015-01-05', '2015-01-04'], dtype='datetime64[ns]', freq=None)
>>> idx.is_monotonic
Out[29]: False
>>> idx = pd.to_datetime(['2015-01-01', '2015-01-02', '2015-01-05', '2015-01-04'])
>>> '2015-01-01' in idx  # This should work
Out[31]: False
>>> pd.Timestamp('2015-01-01') in idx
Out[32]: True
>>> '2015-01-02' in idx
Out[33]: True
>>> '2015-01-04' in idx
Out[34]: True

```
#### Expected Output

``` python
>>> '2015-01-01' in idx
Out[31]: True
```
#### output of `pd.show_versions()`

``` python
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.32-573.7.1.el6.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7.commod
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: 0.7.2
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.1
dateutil: 2.5.2
pytz: 2016.4
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
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: 4.3.2
html5lib: 0.999
httplib2: 0.9.2
apiclient: 1.5.0
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: None
```
