read_excel with sheetname=None or sheetname=[0,1,”Sheet5”] should preserve sheets order
Hello,

I noticed in http://pandas.pydata.org/pandas-docs/stable/generated/pandas.read_excel.html#pandas.read_excel
that `read_excel` can have a parameter `sheetname`set to `None` or a to a `list`

In such a case a dictionary of `DataFrame` is returned.

Maybe it should output an `OrderedDict` of `DataFrame` to preserve sheets order.

Kind regards
