I've implemented a screenshot function of NSView, UIView and GLKView.

Hi there.

I've implemented a screenshot function of NSView, UIView and GLKView.
NSOpenGLView does not support.

* In ```UIView#-(UIImage*)captureImage```, make the image using the ```GLKView#-(UIImage*)snapshot``` method in the case of GLKView.
* I added a ```-(NSString*)captureBase64PngImage``` method to  NSView and UIView. To FrankHelper, this sends a PNG image with the Base64 encoding.

* ```FrankHelper#capture(selector)``` returns to the test code with Base64 decode this data.