Tweak JSHint and JSCS checks.

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.
Run `grunt`.
Merge pull request #13388 from twbs/xhmikosr-js-style

Improve JS style