Added grayscale filter with a bunch of other changes:

 - Improved README file
 - Added more images. Android KitKat everywhere!. Get the list from the AssetManager instead of having the list of filenames.
 - Refactored code for BitmapEffects so it's more extensible now.
 - Get rid of asyncrhonous method calls to apply the effects, since it makes the code much simpler (Let's make it work for now and I can put asyncrhonism back later if necessary).
 - Apply filters to the same Bitmap every time, instead of creating a new Bitmap (from the assets) and applying the filters to it, so we can apply grayscale to a blurred image or reblurry an image.