Horizontal Layout & Storyboard Support

Added support for UICollectionViews which scroll horizontally instead of vertically. Now `TLSpringFlowLayout` will animate and properly layout both vertical and horizontal collection views.

Improved init methods to support initialization from either a Storyboard (`initWithCoder:`) or directly from code (`init`). Both methods call a common `setup` method.

Attempted to fix issue #5 by adding the `prepareForCollectionViewUpdates:` method.

Added a storyboard to the demo project.

Updated details in the Readme.md file.