Run all task spawning through util, to allow for easy hooking.

During debugging, I found it useful to hook all task creation in a
central location, and util::task was the perfect place for it.

r? @pcwalton (or maybe someone else, I'm kinda sending you a bunch of
reviews today because I don't know who better to give them to)

components/canvas/lib.rs

@@ -6,5 +6,6 @@
 
 extern crate azure;
 extern crate geom;
+extern crate "util" as servo_util;
 
 pub mod canvas_render_task;