added support for unnamed (None) axes in to_xxx/read_XXX
to_excel sheet_name defaults to "Sheet1" like in Pandas
added sort_columns and na arguments to read_hdf
added sort_rows, sort_columns and na arguments to read_excel
nicer read_excel code
column titles were inadvertently converted to lowercase
added a few more steps to the notebook to be more explicit
hopefully fix doctests on Python3
added howtocontribute document