Improve target handling

[thomcc]

This fixes #9 and #10.

The complexity is mainly in the fix for #10, to avoiding invoking rustc --version --verbose once per task. It completes somewhat quickly, but we often have a lot of tasks. Ideally we'd just use a Lazy, static ctor, or some other way of doing the same thing we do automatically, but our need to pass in the Project prevents this.

I also improved the documentation for targetDirectory in the fix for #10, since it didn't explain when you might want to use it, and i think the example could be confusing (since there's a release folder inside the target directory).

[thomcc]

I've cleaned up the commit history as well.