cannot truncate a timeseries without sorting it first
#### Code Sample, a copy-pastable example if possible

```
pd.Series(index=pd.to_datetime(['2016-04-15 21:18:04.683115', '2016-04-15 21:18:05.384231',
               '2016-04-15 21:18:05.504638', '2016-04-15 21:18:05.510117',
               '2016-04-15 21:18:05.370865', '2016-04-15 21:18:05.577177',
               '2016-04-15 21:18:06.088652', '2016-04-15 21:18:05.865269',
               '2016-04-15 21:18:05.867652', '2016-04-15 21:18:07.207526'])).truncate(before = '2016-04-15 21:18:03.577077', after ='2016-04-15 22:18:03.577077')
```

I get the error:

> ValueError: index must be monotonic increasing or decreasing \pandas\indexes\base.py", line 2791, in _searchsorted_monotonic
#### Expected Output

```
2016-04-15 21:18:04.683115   NaN
2016-04-15 21:18:05.384231   NaN
2016-04-15 21:18:05.504638   NaN
2016-04-15 21:18:05.510117   NaN
2016-04-15 21:18:05.370865   NaN
2016-04-15 21:18:05.577177   NaN
2016-04-15 21:18:06.088652   NaN
2016-04-15 21:18:05.865269   NaN
2016-04-15 21:18:05.867652   NaN
2016-04-15 21:18:07.207526   NaN
```
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, 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: None
xarray: None
IPython: 4.1.2
sphinx: 1.3.1
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
