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.