BUG: df.plot() with FixedOffset timezones
#### Code Sample, a copy-pastable example if possible

The following example will trigger an exception:

```python
import pandas as pd
import datetime

data = []
now = datetime.datetime.now(tz=datetime.timezone.utc)
for i in range(10):
    data.append(dict(date=now + datetime.timedelta(days=i), data=i))

df = pd.DataFrame(data, index=[x['date'] for x in data])

df.plot()
```

This should fail with the error:

```python
pandas/_libs/tslib.pyx in pandas._libs.tslib.dates_normalized (pandas/_libs/tslib.c:88136)()

AttributeError: 'datetime.timezone' object has no attribute '_transition_info'
```

#### Problem description

The "else" branch in this code will fail on instances of datetime.timezone which don't have a `_transition_info` internal field, which is true with objects like `datetime.timezone.utc`.

The code on master looks like this:

```python
    else:
        trans, deltas, typ = _get_dst_info(tz)

        for i in range(n):
            # Adjust datetime64 timestamp, recompute datetimestruct
            pos = trans.searchsorted(stamps[i]) - 1
            inf = tz._transition_info[pos]

            pandas_datetime_to_datetimestruct(stamps[i] + deltas[pos],
                                              PANDAS_FR_ns, &dts)
            if (dts.hour + dts.min + dts.sec + dts.us) > 0:
                return False
```

The problem is the setting of `inf` and its reference to `_transition_info`, but if I'm reading this code correctly, this is dead code anyway.  With that line removed, this code should work with things like `datetime.timezone.utc` again.

#### Expected Output

I'd expect to be able to plot off timestamps which use datetime.timezone.utc as their timezone.

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

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-27-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.13.0
scipy: 0.19.1
xarray: None
IPython: 6.1.0
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: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
