Added primitive search mechanism in backend.

Began implementing search mechanism for frontend.

Props/Services/Modules/IMetricsManager.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Props.Models.Search;
+using Props.Shop.Framework;
+
+namespace Props.Services.Modules
+{
+    public interface IMetricsManager
+    {
+        public IEnumerable<ProductListingInfo> RetrieveTopListings(int max = 10);
+
+        public IEnumerable<string> RetrieveCommonKeywords(int max = 50);
+
+        public void RegisterSearchQuery(string query);
+
+        public void RegisterListing(ProductListing productListing, string shopName);
+    }
+}