Added the ability to set a shadow to indicators.

[alejandroivan]

Adds these methods to configure shadows to the indicators.

@property(nonatomic) BOOL pageIndicatorShadowEnabled;       // default is NO
@property(nonatomic) UIColor *pageIndicatorShadowColor;     // default is [UIColor blackColor]
@property(nonatomic) CGFloat pageIndicatorShadowRadius;     // default is 1.0f
@property(nonatomic) CGFloat pageIndicatorShadowOpacity;    // default is 0.6f
@property(nonatomic) CGSize pageIndicatorShadowOffset;      // default is {0.5f, 0.5f}