add pipeline.rs, modularized pipelines communicating with constellation
rename engine --> constellation
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
new token-permissions model, and compositor filters paint messages based on id
removed unused macros from main/macros.rs
auto merge of #550 : tkuehn/servo/master, r=metajack

When loading pages, the constellation spawns a new script<-->layout<-->renderer pipeline that loads in the background while scripts continue executing on the currently loaded page.
Implement WebIDL method overloads. Fixes #540.
auto merge of #549 : jdm/servo/overload, r=jdm,metajack
permission bool replaces compositor token
render task no longer copies the layer buffer sets before sending paint messages -- uses an ARC instead
removed unused constellation chan from render task
auto merge of #558 : tkuehn/servo/master, r=metajack

* constellation grants permission to renderers instead of passing tokens
* render tasks wrap layer buffer sets in ARCs
* removed no-longer-used field (constellation chan) from render task struct
Make quadtrees generic
Address pcwalton's comments
Implement progressive rendering
Clean up and annotations
Cleanup after merge
Fix paint permissions issue, clean up