Extract Pastable & Uploadable into separate mixins. 

Has a few BREAKING API changes by doing this, had to change the image, tweet and video blocks to work with the new format. Also, have added a new  element when the block is pasteable / uploadable or droppable. This allows us to easily show / hide controls as before without tying ecverything to droppable.

@ninjabiscuit you should check this out and make sure it meets your requirements before we merge this in :thumbsup: