Added IExchangeService interface

CryptoExchange.Net/Interfaces/IExchangeService.cs

@@ -0,0 +1,13 @@
+namespace CryptoExchange.Net.Interfaces
+{
+    /// <summary>
+    /// Service for an exchange
+    /// </summary>
+    public interface IExchangeService
+    {
+        /// <summary>
+        /// The exchange the service is for
+        /// </summary>
+        public string Exchange { get; }
+    }
+}