ofxiOSVideoPlayer API updates

- replaced getPixelsRef() with getPixels()
- replaced getTexture() with getTexturePtr()
- introduced ofPixels and removed old `unsigned char` pixel arrays.
- replaced pixel order swizzling functions with vImage methods from Accelerate framework (same as in the OSX version of AV video player)
- updated ofxiOSVideoPlayer example with the API changes.
- added Accelerate.framework to iOS template project.
to make it clearer, what i would do is:

```
const ofPixels & getPixels() const{
    return const_cast<ofxiOSVideoPlayer*>(this)->getPixels();
}
```