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