Comply to the new style.
Remove `boss`.
carousel.js: remove unneeded variable assignment.

`slideIndex` is assigned to the same value a few lines above.
Enable `es3`.
Switch to using plain `if/else` statements when there's no assignment.

The minifier outputs the same code anyway, and this has better readability.
Minor style tweaks.
Fix failing tests.
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.
Run `grunt`.