add getShopBySignLocation(Location signLocation) to api

api/src/main/java/com/alttd/playershops/api/ShopHandler.java

@@ -19,6 +19,14 @@ public interface ShopHandler { // TODO finish docs
      */
     Shop getShop(Location location);
 
+    /**
+     * Get the shop at a given location
+     *
+     * @param signLocation Location of the shopSign
+     * @return Shop at the given location or <b>null</b> if no shop is found there
+     */
+    Shop getShopBySignLocation(Location signLocation);
+
     /**
      * Checks whether there is a shop at a given location
      * @param location Location to check