[WIP] Remove Sync and Clone from EventsLoop. Add EventsLoopProxy.

This commit only updates the top-level API to get some early feedback.
None of the platform-specific code has been updated yet. I'm hoping to
get around to this over the next couple days however if someone more
familiar with the windows backend would like to do a PR against this
fork that would be a great help.

Closes #187.

tests/events_loop.rs

@@ -1,10 +0,0 @@
-extern crate winit;
-
-// A part of the API requirement for `EventsLoop` is that it is `Send` + `Sync`.
-//
-// This short test will only compile if the `EventsLoop` is `Send` + `Sync`. 
-#[test]
-fn send_sync() {
-    fn check_send_sync<T: Send + Sync>() {}
-    check_send_sync::<winit::EventsLoop>();
-}