Add support for non-UTF8 CSS stylesheets, with rust-encoding.

Depends on https://github.com/mozilla-servo/rust-cssparser/pull/33 being merged.

Also (unrelated) pass around a base URL for stylesheets.

Adds a new dependency:
Upstream: https://github.com/lifthrasiir/rust-encoding
Servo’s fork: https://github.com/mozilla-servo/rust-encoding

As of this writing, upstream’s master branch targets Rust 0.8, and its rust-0.9-pre branch targets Rust master. Servo uses a Rust version in-between those. I pushed a rust-servo branch to our fork that backports from rust-0.9-pre, and also included some changes that have yet to be merged in upstream pull requests.