add gradientLayer for fillColor

[dgutyanghs]

FSLineChart is awesome, very helpful in my App.
also, I add a gradient color for fill mode.
hope you enjoy!

Thanks!

[ArthurGuibert]

Thanks for the pull request! Is the gradient just for the background of the chart? Is it not something we want to be specific to the app? It wouldn't be hard to add the layer with a zPosition of -1 to be underneath the chart and then we wouldn't have to touch the FSLineChart class.

[dgutyanghs]

Sorry, I hadn't make myself clear.
Why adding a property in FSLineChart.h
@property (nonatomic , strong) CAGradientLayer *gradientLayer;
because I only want to fill the area of fillLayer.path in the function:- (void)strokeChart, not the whole background.