Adding generated images directory for compass sprites

This adds a `generated_images_dir` option, which causes compass will compile sprites to `project/static/img` rather than `project/source/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 project/source/scss/style.scss`.