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.

![ios simulator screen shot jul 28 2014 12 38 35 am](https://cloud.githubusercontent.com/assets/2078653/3717708/58e960aa-162a-11e4-9d38-ea9eb2bb6059.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)