Really adding displayBlanksAs for charts

[tiria]

Adding displayBlanksAs possible values and put it in excel file.
In a chart with blanks values, you can now connect points or define blanks values to zero.

Put plotVisibleOnly in excel file.

[MarkBaker]

Adding $displayBlanksAs = self::DISPLAY_BLANKS_AS_GAP among the existing arguments to the Chart constructor will break backward compatibility, and certainly break the Excel2007 Reader when charts are being loaded..... it would be better to have this new argument at the end

[tiria]

Adding $displayBlanksAs = self::DISPLAY_BLANKS_AS_GAP is necessary to not break the modified Excel2007 writer (reader is not a problem cause already retrieve the good $displayBlanksAs value, but this value was never write with the old Excel2007 writer), if previous default value $displayBlanksAs = '0' is passed to the writer, an error occur cause '0' is not a valid displayBlanksAs value for excel. However, to maintain backward compatibility, I force value to be a valid string for excel chart ('gap','span' or 'zero'), see ST_DispBlanksAs details and dispBlanksAs default values.

Also, I see that i pulled a bad version of PHPExcel_Chart class with a basic error in it (fixed).