Indicator stroke color and width options

Added options to define stroke color (thanks Or Sharir, but I can't find your github account anymore?) as well as stroke width. Allows the creation of simple hollowed out dots:

```obj-c
pageControl.indicatorDiameter = 5.0f;
pageControl.pageIndicatorTintColor = [UIColor clearColor];
pageControl.pageIndicatorTintStrokeColor = [UIColor whiteColor];
pageControl.pageIndicatorTintStrokeWidth = 0.5f;
pageControl.currentPageIndicatorTintColor = [UIColor whiteColor];
```