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.