render circular page indicators with cocos2d 2.x

This pull request makes changes to CCScrollLayer.m, which now renders circular page indicators instead of rectangles with cocos2d 2.x and OpenGL ES 2.0.

It uses very simple vertex and fragment shaders to achieve this effect. The whole row of page indicators is rendered in one OpenGL call, and another one to highlight the active page.

The code has been tested with cocos2d 1.x and cocos2d 2.x, each with retina enabled and disabled.