Move jshint options to .jshintrc

JSHint's configuration has been moved _out_ of the `Gruntfile.js` and
_into_ an external `.jshintrc`.

Note: No new options have been added; only previously existing options
have been migrated. One exception is the `trailing` option which was
excluded, as JSHint does not appear to currently support this option [1].

[1] http://www.jshint.com/docs/options/