Added property to KGNoiseLinearGradientView to allow specification of gradient direction

KGNoiseLinearGradientView provides a property to set gradient direction to 0, 90, 180, or 270 degrees.

typedef enum {
    KGLinearGradientDirection0Degrees, // Left to Right
    KGLinearGradientDirection90Degrees, // Bottom to Top
    KGLinearGradientDirection180Degrees, // Right to Left
    KGLinearGradientDirection270Degrees // Top to Bottom
} KGLinearGradientDirection;

```obj-c
@property (nonatomic) KGLinearGradientDirection gradientDirection;
```