Add remove image method.

Add the possibility to remove an image.

Can be used to cancel uploads and later on remove the image when user selects it.

cc @diegoreymendez 
I think there might be an issue here if the user adds an image outside of our app (e.g. using the media picker on the web).  If the image element does not have an id, the ID selector won't find anything. A good example of this is trying to delete our "cowboy cat" picture in the demo app.

For reference, I just created a post on the web and inserted an image from the media library...this was the HTML (no id):

``` html
<img class="alignnone size-medium wp-image-1156" src="https://bummypublic.files.wordpress.com/2014/12/img_00053.jpg?w=300" alt="IMG_0005" width="300" height="199" /> 
```

We may have to come up with a more sophisticated way to find the image we want to delete OR perhaps create IDs for images that are missing them when the user loads the post into the editor...