Import std::num::NumCast explicitly.

It was removed from the prelude as part of <https://github.com/rust-lang/rust/pull/18827>.
Import std::num::Num explicitly.

It was removed from the prelude as part of <https://github.com/rust-lang/rust/pull/18827>.
Import std::num::Float explicitly.

It was removed from the prelude as part of <https://github.com/rust-lang/rust/pull/18827>.
Merge pull request #57 from servo/rustup_20141124

Import std::num::NumCast explicitly.