Add cargo-init, a cabal-like template generator

This independent module  initializes cargo repositories, much alike the functionality of "cabal init" for Haskell, or "dub init" for D. It allows the user to provide a name and license for their project, and have an instantly-buildable cargo package structure built for them.

cargo-init works both interactively and via command-line arguments.
Did you miss [my comment](https://github.com/rust-lang/cargo/pull/93#discussion_r14333149) about making big long line of `toml`s nicer?

(I guess it needs to be slightly adjusted to include an `if !lib { src_file.write_str("fn main() {\n}") }` too.