RemoteViewsTarget feature requested on #242

Ability to load a bitmap into an ImageView of a Widget using
RemoteViews.

Example of usage on the following gist
https://gist.github.com/pavlospt/00242d5102ac8f76e169
I think `getSize(callback)` is not called when `override()` is used, that's why `SimpleTarget` works, extending it would just reduce your code and may get more benefits later in case `SimpleTarget` is updated. The same code from `SimpleTarget` with `override` would look like this:
```
Glide.load("http://somefakeurl.com/fakeImage.jpeg")
     .asBitmap()
     .fitCenter()
     .override(250, 250)
     .into(new SimpleTarget<Bitmap>() {
         @Override
         public void onResourceReady(Bitmap resource, GlideAnimation<Bitmap> glideAnimation) {
             // Do something with bitmap here.
         }
     });
```
Notice how the size is decoupled from the target, we're free to change the target and not affect the loading itself.