Update better-better-booru.js

* Set arrow navigation to false for now since Danbooru now has its own built in.
* Removed an empty function and added a missing semicolon.
* Updated the listing to work with the new arrow navigation options on Danbooru.
* Updated the getVar function since it couldn't deal with search arguments that were blank (ex: the "tags=" in "http://danbooru.donmai.us/posts?tags=&page=2").