`axis` param for pd.Panel doesn't work
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd
a = pd.Panel({"A":{"a":[1,2,3,4,5], "b":[10, 20, np.nan, 40, 50]})
a.ffill(axis=0)["A"]
a.ffill(axis=1)["A"]
a.ffill(axis=2)["A"]
```
#### Problem description

change to the param `axis` doesn't change the result. It all gives:

     a     b
0  1.0  10.0
1  2.0  20.0
2  3.0  20.0
3  4.0  40.0
4  5.0  50.0

#### Expected Output

     a     b
0  1.0  10.0
1  2.0  20.0
2  3.0    3.0
3  4.0  40.0
4  5.0  50.0

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 32.1.3
Cython: 0.25.2
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: 0.7.9.None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None

</details>
