BUG: reshaping of Panel breaking with datetimeindex
```
In [32]: pd.__version__
Out[32]: '0.18.0rc1+40.gbd1eebb'
```

```
In [23]: dr = pd.date_range('20150101','20150131',freq='T')

In [24]: p1 = pd.Panel(items = dr, data=np.random.randn(len(dr),3,3))

In [25]: p1
Out[25]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 43201 (items) x 3 (major_axis) x 3 (minor_axis)
Items axis: 2015-01-01 00:00:00 to 2015-01-31 00:00:00
Major_axis axis: 0 to 2
Minor_axis axis: 0 to 2

In [26]: p1.swapaxes(0,1)
Out[26]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 3 (items) x 43201 (major_axis) x 3 (minor_axis)
Items axis: 0 to 2
Major_axis axis: 2015-01-01 00:00:00 to 2015-01-31 00:00:00
Minor_axis axis: 0 to 2

In [27]: p1.swapaxes(0,1).iloc[0,0,0]
Out[27]: 2.1388125215371305

In [28]: p1.swapaxes(0,1).to_frame()
TypeError: data type "minor" not understood
```
