Allow transparent images

Load PNGs as ARGB_8888 and JPEGs as RGB_565 preferentially.
Also save ARGB_8888 images to disk cache as PNGs and RGB_565
as jpegs. Fixes some quality issues with transparent images
ExifOrientationParser -> ImageHeaderParser
remove some unused vars in ImageResizer
Add some deprecation warnings and comments

Goal will be to include a more accurate api that
allows you to set only options Glide/the image 
resizer will actually pay attention to in the
future.