Begin creating polymorphic association between Tweet model and TextTweet model
-an ImageTweet model will be added next
Add the ability to create an ImageTweet
-add form to upload an image
-add paperclip gem
-add ImageTweet model, ImageTweetsController
-update routes
-add partial to view an ImageTweet
Add user gravatar
Add the ability for a user to follow other users
Add the ability to unfollow other users