Tweak JSHint and JSCS checks.

Use stricter options and be more consistent across the whole codebase.
Also, make use of JSHint's 2.5.1 `extends` and `qunit` options. This way we set our basis options in js/.jshintrc and override the rest.
Comply to the new style.
carousel.js: remove unneeded variable assignment.

`slideIndex` is assigned to the same value a few lines above.
Minor style tweaks.
Fix failing tests.
Run `grunt`.