Better reverse proxy support

Added "base path override" config option that makes all links and
redirects work with your reverse proxy.
Fixed post config update reload to work properly.
Make redirects and ajax calls use relative pathing.

src/Jackett/Startup.cs

@@ -58,6 +58,12 @@ namespace Jackett
             set;
         }
 
+        public static string BasePath
+        {
+            get;
+            set;
+        }
+
         public void Configuration(IAppBuilder appBuilder)
         {
             // Configure Web API for self-host.