Fix #29: Add `npmVersion` to `node` extension.

 - This is implemented as a special `NpmTask` that installs npm at the
   version specified by `npmVersion` in the `node {}` configuration
   closure.
 - The `setupNpm` task will be skipped if `npmVersion` is not set,
   and works independently of the `download` or `version` options.
Why not just "dependsOn"?