Use ASDisplayNodeScreenScale().

ASImageNode and ASHighlightOverlayLayer were both using
[[UIScreen mainScreen] scale] directly.  Bad.  No cookie.  Use
ASDisplayNodeScreenScale() instead.

(Also, regenerate Xcode project with Xcode 6 and current CocoaPods.)
Remove ASImageNode thread affinity asserts (#57).
Improve ASRangeController dealloc-safety.

(1) We can't size nodes without a delegate, and if our owning controller
has been deallocated, our delegate will be nil.  Handle this more
gracefully.

(2) Our teardown code needs to be executed on the main thread, but it's
conceivable that we might hit refcount 0 on a background queue.  Force
deallocation to occur on the main thread.

Closes #27.