Convert backslashes to forward slashes
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
using ee()

And adding support to this function on versions below 2.6
@2x not as RegEx

There's no need using RegEx on retina checking
removing double slashes without capturing group
Checking for valid URL using filter

instead of a complex RegEx. Native functions are faster.
is_readable instead of fopen
Merge pull request #2 from robsonsobral/develop

part 1 of performance improvements
trim line endings
moving the Exception to the image resize

We need to check if we can't resize the image before throw a exception.
You'll probably wanna change this.
file_exists() just once on isCached
destroying image created by Imagick
to calc dimensions just if there's no cached file
setting out measures earlier
Merge pull request #3 from robsonsobral/develop

part 2 of performance improvements