forward/back navigation with shift+backspace and backspace

script caches last loaded url -- currently no caching policy
naive caching of render layers for near-instant forward/back
handling evicted pipelines is currently broken

src/components/script/html/cssparse.rs

@@ -8,7 +8,6 @@ use std::cell::Cell;
 use std::comm;
 use std::comm::Port;
 use std::task;
-use std::str;
 use newcss::stylesheet::Stylesheet;
 use newcss::util::DataStream;
 use servo_net::resource_task::{ResourceTask, ProgressMsg, Load, Payload, Done};