Panel constructed with a None as a dict value throws an error
Python 2.7.3 (default, Jun 21 2012, 07:50:29)
[GCC 4.4.5] on linux2
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> > > import pandas
> > > print pandas.**version**
> > > 0.8.1
> > > print pandas.Panel(dict(a = None, b = pandas.DataFrame(index=[1,2,3],columns=[1,2,3])))
> > > Traceback (most recent call last):
> > >   File "<stdin>", line 1, in <module>
> > >   File "/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/panel.py", line 219, in **init**
> > >     mgr = self._init_dict(data, passed_axes, dtype=dtype)
> > >   File "/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/panel.py", line 256, in _init_dict
> > >     major = _extract_axis(data, axis=0)
> > >   File "/usr/local/lib/python2.7/site-packages/pandas-0.8.1-py2.7-linux-x86_64.egg/pandas/core/panel.py", line 1396, in _extract_axis
> > >     raw_lengths.append(v.shape[axis])
> > > AttributeError: 'NoneType' object has no attribute 'shape'

same behavior exists in 0.9
added tests and fixes for this case (and the pathological case of constructing with dict(a=None))
