Get text from cell with rich text format.

If a cell with rich text format, i.e. text with font family, font size and so on, the original code will return `None`, seeing test case 900d518.
This pr will extract the cell text from `RichTextRun` nodes according to this http://msdn.microsoft.com/en-us/library/office/gg278314(v=office.15).aspx.