tabular: format date typed cells for humans

 * The tabular renderer for spreadsheets using xlrd was displaying
   date fields as floats, since that is how they are stored in the
   spreadsheet.  xlrd can detect if a field was formatted as a date
   and provides methods to turn these values into python datetime
   objects.  One drawback is that user-local timezone information is
   not available, so MFR will always render datetimes as UTC.

   See:
     http://xlrd.readthedocs.io/en/latest/dates.html
     http://xlrd.readthedocs.io/en/latest/api.html#module-xlrd.xldate

mfr/extensions/tabular/libs/xlrd_tools.py

@@ -34,10 +34,16 @@ def xlsx_xlrd(fp):
             for index, value in enumerate(fields)
         ]
 
-        data = [
-            dict(zip(fields, sheet.row_values(row_index)))
-            for row_index in range(1, sheet.nrows)
-        ]
+        data = []
+        for i in range(1, sheet.nrows):
+            row = []
+            for cell in sheet.row(i):
+                if cell.ctype == xlrd.XL_CELL_DATE:
+                    value = xlrd.xldate.xldate_as_datetime(cell.value, wb.datemode).isoformat()
+                else:
+                    value = cell.value
+                row.append(value)
+            data.append(dict(zip(fields, row)))
 
         header = header_population(fields)
         sheets[sheet.name] = (header, data)