User avatar delegate should returning UIView instead of UIImage

[carmenlau]

In the PR, we only allow user customize the user avatar by returning UIImage.
Ideally, it should be UIView.
#127

[carmenlau]

After discussion, since the avatar UIView is wrapped by JSQMessageViewController, it is difficult to extract and UIImage should be able to satisfy most of the customization use case. Will deprioritize this change before we have real needs.