Added auth/authenticate endpoint

Birdmap/Exceptions/AuthenticationException.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Birdmap.Exceptions
+{
+    public class AuthenticationException : Exception
+    {
+        public AuthenticationException()
+            : base("Username or password is incorrect.")
+        {
+        }
+    }
+}