update library to use JSQMessageAvatarImageDataSource and JSQMessagesAvatarImage. refactor JSQMessagesAvatarImageFactory. add avatarImageView to cell prototype. update unit tests. updates docs. ref #319
check for nil data sources.
fix JSQMessagesBubbleImageFactory docs typos
new helper method on JSQMessagesAvatarImage and unit tests.
update demo for JSQMessageAvatarImageDataSource changes
add TODO
rename JSQMessagesAvatarFactoryTests —> JSQMessagesAvatarImageFactoryTests