Adding generated images directory for compass sprites

This is the mirror pull request from https://github.com/ff0000/generator-red-django/pull/12.

This adds a `generated_images_dir` option, which causes compass will compile sprites to `deploy/static/img` rather than `static/img`.

The `http_*` options are removed as they are not needed because we are using relative asset paths via the `relative_assets` option.

The `force:true` option was added to the grunt task to always have compass compile changes. Before this change, running `grunt compass:build` after `grunt compass:develop` would not result in minified css. Instead, compass would output `unchanged static/scss/style.scss`.

This also adds the `static/lib` folder to the compass import paths. This will also be useful for any other sass libraries installed with bower.

```scss
// before
 @import "../lib/caboose/caboose";
 @import "../lib/caboose/src/caboose/reset";
// after
 @import "caboose/caboose";
 @import "caboose/src/caboose/reset";
```