structural changes to support Iframes
handle pipeline exiting properly
navigation bug fix
switched window.page from *mut Page to @mut Page
removed a call to copy in servo.rc and renamed NavigationContext.navigate ==> NavigationContext.load
remove unused arg from script_task::handle_navigate_msg

remove unnecessary commented-out code
fix borrow check failures by reverting to storing *mut Page in window
auto merge of #628 : tkuehn/servo/master, r=metajack

Completed:
  * Replace pipeline navigation context with frame tree navigation context
  * Reuse script task for same origin iframes
  * Block on size future for iframe reflow
  * Remove global script context
  * Change codegen to use specific page context rather than script context.

Still to do:
  * Compositor's handling of relative frame positions
  * Compositor currently only asks for tiles from outermost frame
  * Window events are forwarded to the outermost frame's script. Should be routed to proper script via constellation.
  * Layout's sending of updated iframe sizes to constellation or size future (initial size only) as reflow is performed