Add linting for .scss files

Try it out by running `npm install` then `node run_grunt.js scsslint`

This adds some (hopefully) sensible rules for writing scss, building on top of the guidance in [the GOV.UK styleguide for CSS](https://github.com/alphagov/styleguides/blob/master/css.md).

- [Documentation of what the options mean](https://github.com/causes/scss-lint/blob/master/lib/scss_lint/linter/README.md)
- [Getting your editor to lint against our config file](https://github.com/causes/scss-lint#editor-integration)

This commit also fixes the errors found by linting:
![screen shot 2014-10-28 at 10 21 13](https://cloud.githubusercontent.com/assets/355079/4806731/54c8754c-5e8c-11e4-8638-0cc74f64bcfd.png)

In the future we could integrate linting into our build process.
It would catch [things like this(https://github.com/alphagov/supplier-submission-portal/blob/master/app/assets/sass/components/_optional.scss#L3). Good thing/bad thing?