BUG: setitem fails on mixed-type Panel4D
Trying to set a value on a mixed-type Panel4D currently fails:

```
In [1]: from pandas.util import testing as tmd

In [2]: p4d = tmd.makePanel4D()

In [3]: p4d['foo'] = 'bar'

In [4]: p4d.iloc[0,0,0,0] = 0
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-bb74b95f7244> in <module>()
----> 1 p4d.iloc[0,0,0,0] = 0

/home/stahlous/pd_dev/pandas/pandas/core/indexing.py in __setitem__(self, key, value)
    119                 indexer = self._convert_to_indexer(key, is_setter=True)
    120
--> 121         self._setitem_with_indexer(indexer, value)
    122
    123     def _has_valid_type(self, k, axis):

/home/stahlous/pd_dev/pandas/pandas/core/indexing.py in _setitem_with_indexer(self, indexer, value)
    356             # indexer here
    357             if (len(labels) == 1 and
--> 358                     isinstance(self.obj[labels[0]].index, MultiIndex)):
    359                 item = labels[0]
    360                 obj = self.obj[item]

/home/stahlous/pd_dev/pandas/pandas/core/generic.py in __getattr__(self, name)
   1927         """
   1928         if name in self._internal_names_set:
-> 1929             return object.__getattribute__(self, name)
   1930         elif name in self._metadata:
   1931             return object.__getattribute__(self, name)

AttributeError: 'Panel' object has no attribute 'index'
```

Fortunately, it looks like a simple fix. If this looks OK. I'll add a note to `v0.15.1.txt` (doesn't have a GH issue number yet).

This fix is needed for my latest attempt at the fillna bug #8395.

tests!
 Ah... yeah. There is a test that highlights this in my fillna bug branch, but I guess that doesn't do any good here. I'll add a test as well to this branch.
 I added in a test which captures this issue. I also added in some tests for the non-mixed-type case since it looks like there weren't any tests of setting by index at all for `Panel4D`.

It looks like this only fixes setting by index for scalar values. Setting by index to a `Series` or `DataFrame` is still broken for mixed-type `Panel4D`. That fix is unfortunately not as trivial. I can work on the more comprehensive fix if you want to hold off merging in this partial fix. If you decide to merge in this PR, I can open an issue for the comprehensive fix.
 keep things in separate PRs 
open issues for things you believe are bugs

it's faster/easier to review s single fix at a time
 Thanks. I added the issue number to `test_setitem_by_indexer_mixed_type` but not `test_setitem_by_indexer` since non-mixed-types were working as expected but just not tested.
 merged via https://github.com/pydata/pandas/commit/1041dff59a3626ecfd311e7ab8ba8502e6214fd6

thanks!
