Re-enable 'unused' config option for jshint.

This was supposed to have been done a long time ago, but got
forgotten about. Most of the issues have been cleared up since
then. Just one small issue remained which I've fixed in the
BoxPosition class.
Don't build lib/stringencoding into the dist.

This file doesn't exist anymore since we moved to getting
text-encoding from npm instead.
Don't ignore bower_components since we don't have any.
Don't build the dist files unless we explicitly want to.

In the normal course of development we should instead rely on a dev
build of vtt.js. This minimizes the possibility of accidentally
building and commiting an official dist while developing.

I've also updated cue2json.js to use the dev build instead of node-vtt's
default page, which will be running an older version of vtt.js.