Panel.ffill ignores axis parameter and fill along axis=1
#### A small, complete example of the issue

``` python
a = pd.Panel({'a':np.arange(15).reshape((5,3)), 'b':np.arange(15,30).reshape((5,3))})
a['a'].loc[1,1] = np.nan

a['a']
Out[36]:
    0     1   2
0   0   1.0   2
1   3   NaN   5
2   6   7.0   8
3   9  10.0  11
4  12  13.0  14

a.ffill(axis=2)['a']
Out[38]:
      0     1     2
0   0.0   1.0   2.0
1   3.0   1.0   5.0
2   6.0   7.0   8.0
3   9.0  10.0  11.0
4  12.0  13.0  14.0
```
#### Expected Output

`a.at['a', 1, 1] == 3`
If you ffill along axis=2, you should get `3` instead of `1`
#### Output of `pd.show_versions()`

Pandas 0.19.0
