add captransaction interface.

src/DotNetCore.CAP/ICapTransaction.cs

+using System;
+
+namespace DotNetCore.CAP
+{
+    public interface ICapTransaction : IDisposable
+    {
+        bool AutoCommit { get; set; }
+
+        object DbTransaction { get; set; }
+
+        void Commit();
+
+        void Rollback();
+    }
+}