Chart title and axis label styling features added to 1.7.9

These two commits add the possibility to change font size for both Chart Title and Chart Axis Label in the Excel2007 writer.

The first commit (eab9a9c) simply adds writing of the 'sz' attribute to PHPExcel_Writer_Excel2007_StringTable::writeRichTextForCharts according to:
* http://www.schemacentral.com/sc/ooxml/e-a_rPr-2.html
* http://www.schemacentral.com/sc/ooxml/t-a_ST_TextFontSize.html

The second commit (09c6f5f) modifies PHPExcel_Writer_Excel2007_Chart::_writeValAx and PHPExcel_Writer_Excel2007_Chart::_writeCatAx in order to use PHPExcel_Writer_Excel2007_StringTable::writeRichTextForCharts instead of duplicating code for writing the a:r element.