UnboundLocalError when setting data to a Panel: "local variable 'mat' referenced before assignment"
Hi,

the following code produces a UnboundLocalError when executed with pandas 0.6.0 / Python 2.7:

```
import datetime
import pandas
timeidx = pandas.DateRange(start=datetime.datetime(2009,1,1),
                       end=datetime.datetime(2009,12,31),
                       offset=pandas.datetools.MonthEnd())
lons_coarse = np.linspace(-177.5, 177.5, 72)
lats_coarse = np.linspace(-87.5, 87.5, 36)
P = pandas.Panel(items=timeidx, major_axis=lons_coarse, minor_axis=lats_coarse)
P[datetime.datetime(2009,2,28)] = np.random.randn(72*36).reshape((72,36))
```

Maybe this is to be expected, but it's definitely not intuitive. Since both `P[datetime.datetime(2009,2,28)]` and `np.random.randn(72*36).reshape((72,36))` have shape `(72,36)`, I would expect this to work.

Until it does, what would be the way to do this?
