Implement trait-based ResourceThreads and clean up related naming issues

Changes include:

- Introduce an IpcSend trait to abstract over a collection of IpcSenders
- Implement ResourceThreads collection to abstract the resource-related
  sub threads across the component
- Rename original ResourceThread and ControlMsg into an unifed CoreResource__
  to accommodate above changes and avoid confusions

components/net/lib.rs

@@ -2,12 +2,13 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![feature(custom_attribute)]
+#![feature(custom_derive)]
 #![feature(box_syntax)]
 #![feature(fnbox)]
 #![feature(fs_time)]
 #![feature(mpsc_select)]
 #![feature(plugin)]
-#![feature(plugin)]
 #![plugin(plugins)]
 
 #![deny(unsafe_code)]