Test and improve pasting

In some situations I had to make some checks for `img` tags so that emoji aren't lost, since they don't have a `textContent` value. 

ex:
https://github.com/metalabdesign/wysihtml5/blob/master/src/wysihtml5/views/composer.js#L117
https://github.com/metalabdesign/wysihtml5/blob/master/src/wysihtml5/dom/remove_empty_nodes.js#L17

Will pasting overwrite emoji if its the only child of a block element? That's sort of how it looks. Haven't tried it, obviously.