Handle writing colors with alpha value

- if the alpha value is less than 1, write additionally the opacity
- fixes #129 (issue 4)

Source/SvgElementFactory.cs

@@ -268,10 +268,15 @@ namespace Svg
                     {
                         attributeValue = "1";
                     }
-
-					descriptor.SetValue(element, descriptor.Converter.ConvertFrom(document, CultureInfo.InvariantCulture, attributeValue));
-					
-
+                    var value = descriptor.Converter.ConvertFrom(document, CultureInfo.InvariantCulture, attributeValue);
+                    if (value is SvgColourServer && ((SvgColourServer)value).Colour.A < 255)
+                    {
+                        // handle alpha values less than 1 by adding an opacity property
+                        var opacity = ((SvgColourServer)value).Colour.A / 255.0;
+                        SetPropertyValue(element, "opacity", 
+                            opacity.ToString("F2", CultureInfo.InvariantCulture), document, isStyle);
+                    }
+                    descriptor.SetValue(element, value);
                 }
                 catch
                 {