BUG: DataFrame.append fails to append tz-aware column to empty DataFrame
This is something that works in pandas 0.16.2 but fails in pandas 0.18.  If you append to an empty DataFrame, and the DataFrame you are appending contains a tz-aware datetime column, the append fails.

``` python
import pandas

df = pandas.DataFrame(columns=['dates'])

df_to_append = pandas.DataFrame({'dates': pandas.date_range('2015', periods=3, tz='utc')})

# Fails here
df.append(df_to_append)
```

Here is the failure I get:

```
Traceback (most recent call last):
  File "/Users/ajenkins/Library/Preferences/PyCharm50/scratches/test.py", line 8, in <module>
    df.append(df_to_append)
  File "/Users/ajenkins/dev/pandas/pandas/core/frame.py", line 4334, in append
    verify_integrity=verify_integrity)
  File "/Users/ajenkins/dev/pandas/pandas/tools/merge.py", line 846, in concat
    return op.get_result()
  File "/Users/ajenkins/dev/pandas/pandas/tools/merge.py", line 1038, in get_result
    copy=self.copy)
  File "/Users/ajenkins/dev/pandas/pandas/core/internals.py", line 4545, in concatenate_block_managers
    for placement, join_units in concat_plan]
  File "/Users/ajenkins/dev/pandas/pandas/core/internals.py", line 4642, in concatenate_join_units
    for ju in join_units]
  File "/Users/ajenkins/dev/pandas/pandas/core/internals.py", line 4915, in get_reindexed_values
    missing_arr = np.empty(self.shape, dtype=empty_dtype)
TypeError: data type not understood
```

The error is because pandas is passing an instance of DatetimeTZDtype to numpy, and numpy doesn't recognize it as a valid dtype.
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: 13a07055dedb0f1d06abf30b185a289220ce5c85
python: 2.7.9.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0+154.g13a0705.dirty
nose: 1.3.7
pip: 8.1.1
setuptools: 20.9.0
Cython: 0.24
numpy: 1.11.0
scipy: 0.15.1
statsmodels: None
xarray: None
IPython: 3.1.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
```
