New feature: profile image view now shows initials when user doesn't have a profile picture

If any user doesn't have a profile picture, his/her name initials will show up in the profile picture area instead.

Also updated the insets of chat bubbles to make them more similar to native chat app.
Screenshot attached.

The `ID` property of `User` is deleted because it's only related to the profile picture name and nothing else. This will give developers more freedom.

![ios simulator screen shot jul 28 2014 1 11 57 am](https://cloud.githubusercontent.com/assets/2078653/3717951/e232224e-162e-11e4-916b-b0edf2e2d676.png)

![ios simulator screen shot jul 28 2014 12 39 11 am](https://cloud.githubusercontent.com/assets/2078653/3717707/588af916-162a-11e4-978a-a8aa5bb1ba7b.png)
I think `ID` is important. You might not see it now, but it helps you distinguish between users. Also, when you send a message to a user, you have to tell the server which user you're sending it to. You can use `ID` for that. So, I think `ID` should be added back.