_Work in progress. Do not merge._

This is a difficult problem to generalize for all layouts. I was discussing this with Marc Haisenko, and it clarified a couple things for me.

There should probably be two types of reordering. I'll paste what Marc aptly wrote:

• Displacing: a placeholder (or empty view) is drawn where the drop will occur, the other views are pushed away.
• Marking: a marker is drawn in such a way that no existing items need to move. For example, try to move an item in Xcode's file tree: a blue marker is drawn where you'd like to drop. This also detects into account on which level you want to drop.

To me, it seems like the default type here should be displacement. The reasoning for that is that drawing a marker can really only be generalized to linear layouts. Displacement should theoretically work for all types of layouts.

There are many components that need to go into this. My proposal is this.

JNWCollectionViewReorderableLayout
This will be a subclass of the regular layout. This would encapsulate many of the patterns that are in common with all types of drag and drop operations. Any layouts that want to opt-in to reordering can then subclass this class instead of JNWCollectionViewLayout. This class will be responsible for handling all aspects of both marker and displacement-based layouts. That means that in the case of displacement-based layouts it will take any layout attributes it receives from any subclasses and will modify those as necessary to displace elements.

Now here come the more interesting questions that I haven't figured out yet.

1. What should be responsible for auto-scrolling the collection view? The layout? Should it be forwarded to the collection view, which it can then handle itself?
2. We need to add onto the existing delegate methods to ask the delegate whether it wants to allow the proposed drop, and also to inform it that a drag and drop happened.
3. How will this be synced with the data source? Ideally this should co-exist with APIs for inserting / deleting / moving items.
4. How can we allow customizability of the displacement animations? Should this be part of my grand plan for allowing custom animations for specific events in the layout (e.g. insertion, deletion, moving)?

(Fixes #11)