Add doc and doc-$(crate) targets to the Makefile using rustdoc.
Escape HTML in the cors.rs documentation.

Rustdoc passes the code straight through, leading to poor generated HTML.
Merge pull request #2923 from Ms2ger/cors-escape

Escape HTML in the cors.rs documentation.
Build docs on Travis and upload them to GitHub Pages.

http://servo.github.io/servo/servo/index.html
Merge pull request #2922 from servo/rustdoc

Build rustdoc docs on Travis and upload them to GitHub Pages.
Fix an unnecessary_parens warning.
Merge pull request #2926 from servo/unnecessary_parens

Fix an unnecessary_parens warning.
Port `id` attribute to use atom.
Use atom for comparision of id selector.