Reading NA values properly from XLS files
From pystatsmodels mailing list:

"I've got an excel file created from Paste Values that when read with pandas.ExcelFile returns int 42 for #N/A values.

Looking at http://stackoverflow.com/questions/4928629/xlrd-excel-script-converting-n-a-to-42 it would seem I'd need to be able to access the xlrd cell type rather than the value to interpret as NaN. Is there a way to do this?"

As suggested by the stackoverflow article, we should use the error_text_from_code dict in the xlrd module to convert error codes to the proper text.
