Fix Compass task

This commit fixes a problem with the `compass` task being executed
on `production`. Before this, the `imagemin` task was being executed
after the `compass` task, causing `compass` to fail because it couldn't
find the images under `/dist/images/` (those images are copied during
`imagemin`).

To reflect this change, some styles using Compass sprites has been added.
Add styles.scss reference to README file