Fixed type for gridOptions.borderWidth and also changed it as optional parameter.

See: https://github.com/flot/flot/blob/master/API.md
----
> "borderWidth" is the width of the border around the plot. Set it to
> 0 to disable the border. Set it to an object with "top", "right",
> "bottom" and "left" properties to use different widths.

flot/jquery.flot.d.ts

@@ -46,7 +46,7 @@ declare module jquery.flot {
         labelMargin?: number;
         axisMargin?: number;
         markings?: any;             //array of markings or (fn: axes -> array of markings)
-        borderWidth: number;
+        borderWidth?: any;          // number or width object
         borderColor?: any;          // color or null
         minBorderMargin?: number;       // or null
         clickable?: boolean;