Dynamic cell identifiers

This should address #133. I'm dynamically determining the cell identifier based on the cell features, while keeping intact the user's specified cell identifier class.

So if a user specifies two cells with image as `imagecell` and one has a subtitle and the other doesn't, they'll be changed to `imagecell-image` and `imagecell-image-subtitle`.

**I HAVE NOT TESTED PERFORMANCE**. It may be worth it to see what kind of performance impact this has on cell reuse and scrolling, but I suspect that as long as you're only using a few different types of cells, it should be OK.