Add the ability to drag & drop to rearrange items in the collection view

[jwilling]

If this is to be done correctly, the API will not be a simple one to implement. We can't assume that the layout is going to be linear, so there needs to be some way to tie in with the layout.

I'd love to hear some thoughts on this from the folks that are using this library right now. Anyone up for me implementing this before 1.0, or is this better suited for a 1.1 release so we can make this public a bit sooner?
// @mdiep @CodaFi @tonyarnold

[CodaFi]

I've got it. I can submit my branch today. It's definitely rough, but it's a start.

[cduruk]

While I think this would be nice to have, it's not something that's direly needed for us. Definitely voting 1.1.

[CodaFi]

@cduruk You'd be surprised how many people have been asking for Drag 'n Drop support. Even among Apple classes it's a weird API to get right.

[jwilling]

This is a tough call. I really do want to get this released as soon as possible, and I don't see drag and drop being viable in the (super) near future. I wanted to push for a release this week, but I've been dealing with some personal issues that are taking higher priority right now. I think I'll leave this slated for 1.1, unless there is a convincing argument otherwise.