Partial support for UITableViewHeaderFooterView

[ZevEisenberg]

Caveat: this won't work if you use it for cells and headers, because the cache will collide. I'm using it for headers only, so it works in my case, but before it's merged, it should probably have a separate internal cache for headers, supplementary views, etc.

[ZevEisenberg]

Just realized that we don't necessarily need to add internal support for multiple caches, since you could easily create a separate RZCellSizeManager instance for each set of index paths for a different view type. It might still be a good idea to add this support, but you can still use it with multiple view classes without changing the code.