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)
No need to set `hidden` here as it's done during cell configuration.