Fix floaring header gradient in FF and settings images

No issue

- Corrects the syntax used for the floating header gradient so it now gets prefixed correctly for Firefox
- Removes padding from `<button>` elements (fixes padding around images in buttons)
- Adds display: block; to settings images to prevent a Webkit bug which adds space under the image
Apply JSHint to test files.

closes #2666
- Add new jshint:test task
- Fix jshint errors
Merge pull request #3953 from halfdan/2666-jshint-test

Apply JSHint to test files.
Merge pull request #3952 from PaulAdamDavis/fix-what-libsass-broke

Fix floating header gradient in FF and settings images
fixes #3936