Clears metadata on ImageMagick

GD is already removing the metadata, but ImgMagick wasn't. Maybe
somebody wnt to keep the copyright info of the image.
ImageMagick was using its default quality setting

The default quality setting of ImageMagick is 87. Maybe all default
quality settings could be 87.
Using progressive JPEG

Progressive JPEGs is almost always smaller
Using CONSTANTS instead of hardcoding permissions
IMG tag on XHTML style for backward compatibility
Merge pull request #1 from robsonsobral/develop

Some little improvements, specially on ImageMagick