ERR: validate partial string indexing with tz-aware end-points
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import dateutil.parser

df = pd.DataFrame(index=pd.date_range('2016-01-01T00:00', '2016-12-31T23:59', freq='T'))

df[
    "2016-01-01T00:00-02:00"
    :
    "2016-01-01T02:03"
]  # returns 124 entries

df[
    dateutil.parser.parse("2016-01-01T00:00-02:00")
    :
    dateutil.parser.parse("2016-01-01T02:03")
]  # returns 4 entries

df[
    pd.Timestamp("2016-01-01T00:00-02:00")
    :
    pd.Timestamp("2016-01-01T02:03")
]  # returns 4 entries
```
#### Problem description

The current behavior is that the time zone information is ignored without any warning when providing strings.

#### Expected Output

Either use the time zone information and only return the four desired entries OR warn the user about the fact that the time zone information is not used

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 32
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.12.1
scipy: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
