Resetting Index on slice
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
df = data_index[data_index['ntorsions'] == 2]

```
#### Problem description

When slicing a dataframe, the index is not reset by default.  This becomes an issue if you want to output that dataframe, combine that dataframe with other dataframes (good luck with that), or output the dataframe without two index columns. 

Fixing this will not break code in the wild.

#### Expected Output

Index being correct - without the need to manually call reset_index over and over again. This is much more intuitive to end users.

-> At end of slice, call reset_index(drop = True) on the returned dataframe or current dataframe if you are slicing in-place. 

#### Output of ``pd.show_versions()``

<details>

loaded rc file /Users/jadolfbr/.matplotlib/matplotlibrc
matplotlib version 1.5.1
verbose.level helpful
interactive is False
platform is darwin

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 9.0.1
setuptools: 20.3.1
Cython: None
numpy: 1.11.1
scipy: 0.13.0b1
statsmodels: 0.6.1
xarray: None
IPython: 4.1.2
sphinx: None
patsy: 0.4.0
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
