Development dependencies

Development dependencies are included when building or testing the current package, but not packages that depend on it:

```toml
[dev-dependencies.hamcrest]

git = "https://github.com/carllerche/hamcrest-rust.git"
```
I find that match statements like this are better formatted with an early return:

```rust
let dependencies = match new_deps {
    Some(ref d) => d,
    None => return Ok(())
};
/* ... */
```

Saves two levels of indentation!