BUG: Panel resample not working with loffset
from the ML: https://groups.google.com/forum/#!topic/pystatsmodels/bCatleGJ4UU

```
In [15]: p = Panel(randn(10,100,5),major_axis=pd.date_range('20130101',periods=100))

In [16]: p
Out[16]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 10 (items) x 100 (major_axis) x 5 (minor_axis)
Items axis: 0 to 9
Major_axis axis: 2013-01-01 00:00:00 to 2013-04-10 00:00:00
Minor_axis axis: 0 to 4

In [17]: p.resample('M',axis=1)
Out[17]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 10 (items) x 4 (major_axis) x 5 (minor_axis)
Items axis: 0 to 9
Major_axis axis: 2013-01-31 00:00:00 to 2013-04-30 00:00:00
Minor_axis axis: 0 to 4

In [18]: p.resample('M',axis=1,loffset='-10D')
AttributeError: 'Panel' object has no attribute 'index'
```
