auto merge of #918 : brson/servo/longcat, r=metajack

This fixes layout under Firefox and increases the frame rate for smoother animation.
Pipeline create is now wrapped in a task that supervises the tasks
created by script, rendering, and layout. When any of those fail,
the pipeline is removed from the FrameTree and a new one is created
with a failure HTML file.

Additionally, the top-level Makefile is changed to store debug info.
Transitioned to about:failure instead of having the URL in
Constellation.
Fix formatting issues.
Remove code duplication in finding FrameTrees with a given pipeline id.
Make Element.set_attr require an AbstractNode so we can always downcast.
Remove unused old bindings helpers.
Make Window store an @Page instead of a *Page and remove a bunch of unsafe code.
auto merge of #914 : jdm/servo/unsafepage, r=jdm,metajack

Haven't figured out what to do about page_from_context yet, but this is a big improvement.
Remove a stray space. Clean up awkware if result...result construct.
auto merge of #917 : larsbergstrom/servo/task_failure, r=metajack

Pipeline construction now creates a task that supervises all of the child tasks created by script, render, and layout. If any of those fail, it sends a message back to the constellation, which re-creates the pipeline and threads it into the appropriate spot in either the FrameTree or set of pending frame updates.

Also, turn on debug_info for symbols in the Makefile.
auto merge of #920 : jdm/servo/setattrabstract, r=metajack

r? @metajack
Goodbye, Andreas.
auto merge of #923 : jdm/servo/itried, r=larsbergstrom
Fix a dynamic borrow error when executing timer callbacks.
auto merge of #926 : jdm/servo/pagewhoops, r=metajack

r? @metajack
wip