update rust and dependencies

Depends on my patch to persistent here:
https://github.com/iron/persistent/pull/27

$ rustc -v && cargo version
rustc 0.12.0-nightly (43fd61981 2014-09-22 23:50:30 +0000)
cargo 0.0.1-pre-nightly (a54cb70 2014-09-23 01:34:57 +0000)
README formatting