E.Naumovich - 2014-01-13

I found some issues:
1. If invalid (1/0, etc.) double appears in "double" cell, it lead to error in file. I not found easy way to correct it more or less properly.
2. Empty cells can not keep format. I found easy patch based on the usage of the empty "string" cells, I can submit it but I do not know how.
3. Behavior of the cell background color is obscure. The combination of the background and foreground colors leads to errors in file. Usage of the any background color without foreground one leads to black filling of the cell. I suppose that background color can not be assigned as (A)RGB.