Slicing DatetimeIndex should be timezone aware
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import numpy as np
import pandas as pd

idx = pd.DatetimeIndex(start='2018-12-02 14:50:00-07:00', end='2018-12-03 03:11:00-07:00', freq='1min')

df = pd.DataFrame(np.random.randn(len(idx), 1), index=idx, columns=['A'])

df['2018-12-02 21:53:34+00:00':'2018-12-02 22:53:54+00:00'].head()
#                                  A
# 2018-12-02 21:54:00-07:00 -0.647961
# 2018-12-02 21:55:00-07:00  0.112683
# 2018-12-02 21:56:00-07:00 -1.221521
# 2018-12-02 21:57:00-07:00  0.052644
# 2018-12-02 21:58:00-07:00  1.799572

```

#### Problem description

When slicing a DatetimeIndex using a timestamp that has a timezone, the timezone is not used as part o the comparison.

#### Expected Output

```python
df[(df.index >= '2018-12-02 21:53:34+00:00') & (df.index < '2018-12-02 22:53:54+00:00')].head()
#                                  A
# 2018-12-02 14:54:00-07:00  0.578494
# 2018-12-02 14:55:00-07:00  0.170471
# 2018-12-02 14:56:00-07:00  1.078600
# 2018-12-02 14:57:00-07:00  0.376983
# 2018-12-02 14:58:00-07:00 -1.295243

```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.17.0-3rodete2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: None
pip: 18.1
setuptools: 40.6.2
Cython: None
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
