Update release notes and authors file for 0.12.0

The 0.12.0 release is nigh. I've dug through the commit log looking for the most interesting stuff to add to the release notes, but I'm positive I've missed things that should be mentioned.

The [detailed notes](https://github.com/rust-lang/rust/wiki/Doc-detailed-release-notes) on the wiki also need love.
btree was rewritten