Post Upload Progress Notification Improvements

Improves post upload notification by showing upload progress of media items.

Check out `CountingOutputStream` to see how we track the uploaded bytes.

Also fixes a few issues:
 * Fixes notification icon size, #1704
 * Reorganizes code in `PostUploadService`, #1185. But I think I really just put lipstick on a pig ;)

Video of the upload progress: https://cloudup.com/casufV3XXxN
Rather than using a hard-coded size, how about using [notification_large_icon_height](http://developer.android.com/reference/android/R.dimen.html#notification_large_icon_height)?

```
int largeIconSize = context.getResources().getDimensionPixelSize(android.R.dimen.notification_large_icon_height)
String resizedUrl = PhotonUtils.getPhotonImageUrl(iconUrl, largeIconSize, largeIconSize);
```