Added the ability to specify the image resolution and various other fixes an enhancements

Here are a few fixes and enhancements, that I've accumulated while integrating this great component into a new app. Some of them might be useful for a broader audience, so feel free to merge this in or just cherry pick stuff that seems useful. 

The main improvement is the ability to efficiently capture hi-resolution images (with the appropriate asynchronous GPUImage calls and fallbacks) and to specify the exact desired size of the final images (or to get the maximum possible size if not specified).