Some of the previously defined error names have changed to give
a more consistent interface. Also added a lxw_strerror() function
to decode the error to a string.
Issue #49.
Renamed new_workbook() to workbook_new() and new_workbook_opt() to
workbook_new_opt() for consistency.
The older function names are still availble but deprecated.
Renamed worksheet_set_column() function to worksheet_set_column_opt()
for consistency with current and future APIs. The worksheet_set_column()
function is now used without the option.
This is a backward incompatible change.
