WIP lesscss integration. #112

don't comment on surrounding changes (config, etc). i just wanted this branch to be testable out of box
i [asked](https://github.com/mrclay/minify/pull/112#issuecomment-56267716) the TODO in initial PR, did not get response from you:

    Does Minify have a way to enforce cache invalidation by itself?
    i've passed "0" as isValid call, as lessphp itself invalidates the cache.