Prefer usemin to configure cssmin workflow

In development ('grant server') the user needs actually to reference the css, otherwise other files in app/styles (bootstrap.css just to name one) other than main.css will not be used.
An @import url(bootstrap.css) in main.css is not the best thing because this will be included in the concatenated output file.

So IMO a "usemin" configuration like this is more appropriate.

    <!-- build:css styles/main.css -->
    <link rel="stylesheet" href="styles/main.css">
    <link rel="stylesheet" href="styles/bootstrap.css">
    <!-- endbuild -->