v2 questions / notes for migration guide

Howdy. I'm going through and updating our v1 implementation and I've got a couple of notes. tl;dr: v2 is looking great.

- No way I can find to override or customize the dismiss button. Would be great to have an appearance proxy for the image name if we can't just set it directly
- No simple way to constrain the image in a BLTNPageItem. For example if I set a photo as the image, it will expand to almost full screen. I have experimented with adjusting the constraints in onDisplay, which works OK
````self.imageView?.heightAnchor.constraint(lessThanOrEqualToConstant: 128).isActive = true````
I also tried creating a custom UIImageView, wrapping it, and adding it via makeViews... also worked ok. Maybe ImageView needs a BLTNInterfaceBuilder helper with some constraints arguments?
- makeViewsUnderImageWithInterfaceBuilder() is not called if the image is not set. There is a logic to this, but it's not mentioned anywhere and i had to debug through to figure this out. I think it'd be better to always call it
- Should objects based on_PageBLTNItem_ override tearDown() to nil out custom UI elements? Our v1-based custom Pages did this but I don't know if it was / is necessary.
- Migration guide should mention presentationHandler and onDisplay()
- The Readme.md link to _Creating a Custom Item_ is broken. It also suggests subclassing _BLTNItem_, but based on reading headers, I think it should suggest _BLTNActionItem_, right?
- BLTNPageItem.makeHeaderViewsWithInterfaceBuilder typo. It says "The interface builder used to create the title."