Relax dependency on spree_core to ~> 2.4
Admin Menu: add calculator icon to POS tab.
Namespace admin routes. DRY
JavaScript: only execute when #pos-tabs is found on the page.

Simple hack to make sure spree_pos.js doesn’t needlessly run on every backend page.
Created admin/pos/index action to list incomplete orders.
Admin::Orders: update sidebar POS link to match Spree’s style.
Admin::POS#show: cleaned up page title and added standard order sidebar.