resource(".xls") -> Excel

adds into(DataFrame, Excel) using pd.read_excel
From my experiments, `pd.read_excel` properly detects if the cell value has a `ctype` equal to the value of `XL_CELL_DATE`. I basically just let `read_excel` do its thing. The interesting/bizarre thing is that a cell will have the this date flag set when Excel decides to set it, based on an inference of what you put in the cell. If we ever decide to handle this ourselves, we will have to read the `ctype` of a cell, and convert the underlying float/into to a proper date using `xlrd.xldate_as_tuple` and then pass that to `datetime.datetime`. (copying this to the proper issue for historical notes)