using temp file to storage minified results
add a comment of reaching condition
won't minify any more if the response was ended
fallback to normal response when cache cannot be accessed or cache content is not valid
leave OS FileSystem to determine whether to optimize file reading
dropped support of parsing .sass file ( https://github.com/andrew/node-sass/issues/12 )
use cssmin instead of uglifycss
re-arrange README
added change log and update version