decouple script from compositor
communicate via layout
refactor channel wrappers from *Task --> *Chan

fix merge fallout
fixed docstring for create_compositor_task
*Chan.send() convenience functions
profiler refactor to print every period if new data has arrived
refactor compositor interfaces \--> RenderListener + ScriptListener; AttachCompositorMsg added to render_task::Msg

updated to reflect comments
auto merge of #517 : tkuehn/servo/master, r=metajack

communicate via layout
refactor channel wrappers from *Task --> *Chan
Propagate debug flag in Makefiles.
Adding renderer drawing category to profiler.
Warning police.
Refactor renderer into single function.

This removes the task per tile rendering and instead renders tiles
serially. This also unwraps the rendering into a single function so that it's
much clearer.