Recycle list rows in FeedAdapter

Simple merge request, but transitions the feed adapter over to doing what's described in the comment for convertView [in this hard to find piece of documentation](http://developer.android.com/reference/android/widget/Adapter.html#getView(int, android.view.View, android.view.ViewGroup\)).

Nice thing about this is that it'll:

1. reduce number of layout creations (one of the most expensive things you can do in Android)
2. works out of the box with Picasso, as stated under the first feature [on this page](http://square.github.io/picasso/) meaning that you won't randomly get the wrong image in your row if the user scrolls really quickly.

I was also going to implement a [ViewHolder pattern](http://developer.android.com/training/improving-layouts/smooth-scrolling.html#ViewHolder) but that's really premature optimization at this point.