can't subtract utc datetimes from datetime.utcnow()
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
df = pd.DataFrame()
df["time"] = pd.date_range(start="2017-01-01", end="2018-01-01", freq="D", tz="utc")
df["now"] = pd.datetime.utcnow()
df["now"] - df["time"] #raises ValueError: Incompatible tz's on datetime subtraction ops
```
#### Problem description
Calling `df["now"].dt.tz` returns `None`, meaning that `pd.datetime.utcnow()` has not been explicitly localized to utc. Since `pd.datetime.now()` is tz-naive, it is useful to have a similar function which returns the current time with timezone localization for operating on tz-aware datetimes.

#### Expected Output
`df["now"].dt.tz` returns `<UTC>`
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.14.0
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
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: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
