BUG: Interval can contain end points with different time zones
#### Code Sample, a copy-pastable example if possible

```python
In [2]: left = pd.Timestamp('2017-01-01', tz='US/Eastern')
   ...: right = pd.Timestamp('2017-01-02', tz='US/Pacific')
   ...: iv = pd.Interval(left, right)
   ...:

In [3]: iv
Out[3]: Interval('2017-01-01', '2017-01-02', closed='right')

In [4]: iv.left
Out[4]: Timestamp('2017-01-01 00:00:00-0500', tz='US/Eastern')

In [5]: iv.right
Out[5]: Timestamp('2017-01-02 00:00:00-0800', tz='US/Pacific')
```
#### Problem description
An `Interval` can contain endpoints with different time zones.

#### Expected Output
I'd expect this to raise.

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

<details>

INSTALLED VERSIONS
------------------
commit: 88ab6934e78117359719eb09c7d580906155575d
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.22.0.dev0+247.g88ab693
pytest: 3.1.2
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.1
scipy: 0.19.1
pyarrow: 0.6.0
xarray: 0.9.6
IPython: 6.1.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: None
html5lib: 0.999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: 0.1.0
pandas_gbq: None
pandas_datareader: None

</details>
