Issue/201 insert image at caret position

Fixed [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/201) and [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/293).  Also shows the image immediately after inserting it into the post and before the upload is complete.

/cc @bummytime 
Is this the image we were supposed to use for the placeholder? (I can't remember)