fillna can't process np.float16 type
#### Code Sample, a copy-pastable example if possible
```
p_s = pd.Series(data=[47.97,   np.NAN,  np.NAN, 47.16, 47.97], dtype=np.float16, 
                index=pd.DatetimeIndex(['2015-07-24', '2015-07-25', '2015-07-26', \
                                                          '2015-07-27','2015-07-28']))
p_s
Out:
2015-07-24    47.96875
2015-07-25         NaN
2015-07-26         NaN
2015-07-27    47.15625
2015-07-28    47.96875
dtype: float16
```
#### Problem description
p_s.fillna(method='ffill') 
_get the error:_
**ValueError: Invalid dtype for pad_2d [float16]**
_But:_
p_s.astype(np.float64).fillna(method='ffill')
_or:_
p_s.astype(np.float32).fillna(method='ffill')
works well:
2015-07-24    47.96875
2015-07-25    47.96875
2015-07-26    47.96875
2015-07-27    47.15625
2015-07-28    47.96875
#### Expected Output
p_s..fillna(method='ffill')
Out:
2015-07-24    47.96875
2015-07-25    47.96875
2015-07-26    47.96875
2015-07-27    47.15625
2015-07-28    47.96875
dtype: float16
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
1) On WINDOWS
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.14.0
scipy: 1.0.0
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

2) On UBUNTU 16.04
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-108-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: ru_RU.UTF-8
LOCALE: ru_RU.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: 1.0.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
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: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
