add HAML-Lint (gem, config file, cider spec)
styleguide: add test, fix errors

- per-element-screenshots are shall hashed and checked
styleguide: generate shasums
styleguide: add generated shasums
styleguide: enable test for all elements
styleguide: remove hover effects (for predictable screenshots)
styleguide: keep screenshots in cider (debug)
styleguide: generate shasums (all elements)