auto merge of #720 : alexcrichton/cargo/no-more-plugins, r=brson

It's looking more likely like plugins will not make it into the stable channel
of Rust, so this commits removes Cargo's personal dependence on the two
plugin-based pieces of functionality it was using:

1. Uses of the `regex!` macro now go through `Regex::new`.
2. Uses of the `docopt!` macro now go through `deriving(Decodable)` instead.

diff --cc src/cargo/core/resolver.rs
Simple merge