Port selector matching to use atoms.

Removed the Lowercase new type. This does mean that this
code path is slightly slower for now. This is because
find_equiv() doesn't work with atoms in hash tables, due to
the hash value being different than that of a string. However
it also means the removal of some unsafe code, and a better
long term solution will be to precache the lower case versions
as atoms.
Make Attr::owner immutable.

Nobody needs to change the element it's associated with, so there's no reason
to use a Cell here.
Document.getElementById() should return null if the passed argument is empty.
Merge pull request #3032 from saneyuki/id

Document.getElementById() should return null if the passed argument is empty
Implement CanvasRenderingContext2D.canvas.
Merge pull request #3034 from saneyuki/canvas

Implement CanvasRenderingContext2D.canvas.
Using NodeFlags ctor to set InEnabledState when needed
Only run configure in the Rust submodule when we actually use it.
Merge pull request #3036 from servo/no-configure-rust-submodule

Only run configure in the Rust submodule when we actually use it.
Merge pull request #3035 from brunoabinader/fix-node-ctor

Using NodeFlags ctor to set InEnabledState when needed; r=Ms2ger
Run Travis-CI build on pushes to all branches, not just master.

We can add a blacklist (rather than the previous whitelist) as needed
when we get newbors running.
Merge pull request #3043 from servo/travis-all-the-branches!

Run Travis-CI build on pushes to all branches, not just master.
Have http://servo.github.io/servo/ include docs for the Rust version we use.

Closes #3038.
Merge pull request #3045 from servo/rust-doc-the-vengance-of-the-return

Have http://servo.github.io/servo/ include docs for the Rust version we use.
Move documentation to http://doc.servo.org

[skip ci]
Merge pull request #3046 from servo/doc.servo.org

Move documentation to http://doc.servo.org
Fix rustdoc comment syntax
Get rid of fontconfig warnings for linux travis runs.
Merge pull request #3049 from glennw/fc-warnings

Get rid of fontconfig warnings for linux travis runs.
Merge pull request #3047 from mbrubeck/doc-comment

Fix rustdoc comment syntax