Fix background edge blur

Closes #5.

We now make ```_backgroundImageView```'s frame be 1.2x the size of the ```parentView```'s frame so that ```parentView``` sits centred inside the image view. We then renderInContext the ```_backgroundImageView``` (which contains the ```parentView```), not ```parentView```. This will allow the ```parentView``` to be blurred beyond its bounds.

We then account for this extra scaling of ```_backgroundImageView``` in the show/hide functions.