exit the correct evicted pipelines
find_mut ==> find. it's only mut because of the data type iterated over
script task only exits when the root pipeline exits
split script_task::ExitMsg into WindowExitMsg and PipelineExitMsg
auto merge of #966 : tikue/servo/master, r=metajack

Fixes #967 and #965 

This has been wrong for a long time. Previously, only the pipeline associated with the root frame evicted would be shut down. 1) It shouldn't necessarily be closed, because there could be references to it still in the navigation context, and 2) Presumably none of the children pipelines of the root frame were ever exiting.

It's hard to test this right now because #965 covers up other pipeline exiting issues, but when that's fixed, a pathological case in which things would have broken down would be:

1) Load a page with an iframe that contains a link
2) Click the link
3) Press backspace to navigate back
4) Navigate to any new page, at which point the forward page would be evicted from the navigation context, and the outer frame's pipeline would be shut down improperly.
5) Press backspace, at which point there is no longer a pipeline for the old page, because it was shut down prematurely. Presumably this would cause a crash.

I also changed the FrameTree function ```find_mut``` to ```find``` because find_mut implies it's doing something to cause mutability, but the mutability is caused by the type of object being iterated over, nothing else.

Additionally, script was exiting completely when receiving an exit message. Instead, it needs to handle exit messages according to who sent it. It should only close the subframes of the frame whose pipeline sent the exit message. This is now fixed.

Inexplicably, script was also closing the compositor upon receiving an exit message. This doesn't seem like it'd ever be the right thing to do. *Edit: this is _only_ the right thing to do when received from the window.* I've fixed that. I don't think anyone shuts down the compositor now. *Edit: the script shuts down the compositor only when receiving an exit from the window.*