Add setting to hide avatars

All of the avatars are the grey default guy for me, so I don't want it taking up space. I understand that adding settings = bad though, so if you don't want the feature that's fine. It's just something I wanted and a chance for me to start getting familiar with the code.

If you're interested in merging, I need to add a couple of things:

1. A listener to refresh the view when the setting is changed. Not sure how to do that at the moment, but I can work on that.

2. The avatar is shown in multiple places (e.g. when you start a new message and enter a number), so it should probably be hidden everywhere when the setting is enabled.

3. There should probably be a new way to pull up that contact window or something. It's not really something I use, but there might be people who want to hide the avatar but do want that ability.