Metadata extractor

This branch adds the EXIF-Metadata-Parser to the gallery. Therefore, I changed the following things:
* Added thumbnail option to DisplayImageOptions, so whenever you want to load thumbnails, you have to specify it in the options first

* Added code to the LoadAndDisplayImageTask so that if thumbnails are desired, they are stored with an additional ending: 
``` java
if (options.isThumbnail()) {
      imageFile = discCache.get(uri + ".thm");
    } else {
      imageFile = discCache.get(uri);
    }
```
This way it is possible to keep thumbnails as well as the full resolution pictures cached.

* Added OnLoadingUpdate method to ImageLoadingListener to track download progress of large images.