Generate minified filename based on md5 hash

[joeke]

At the moment the minified filename is generated based on the latest file modification time of the files from the group. But when using imports inside a css file, the modification times of those files are not checked so the minified file is sometimes not updated correctly. Using the md5 hash from the minified file should solve this.

[mindeffects]

Would be an awesome feature! Especially when you work with frameworks like Zurb's foundation, you have to deal with many @import and having checked all of those automagically would be a dream!
Changed a fd setting? No, problem! SCSS parsing done.