ENH: Allow DataFrame.update to work on non DataFrame object if coercible
On issue #1988, a new Panel.update function should accept a dict of DataFrames.  Right now, DataFrame.update can't accept a dict of Series (or a dict of arrays for example).  To me, it wouldn't make much sense for Panel.update to have the functionality while a DataFrame does not.

Currently:

```
>>> df.update(s) #s is a dict of arrays or of Series
...
AttributeError: 'dict' object has no attribute 'reindex_like'
```
