Support writing output to stdout by using out=stdout.

For integration with other (command line) tools it would be nice to write output of r.js directly to **stdout** instead of a file (issue #474). In order to do this one can set *out=stdout*, e.g.:
```bash
node r.js -o name=main baseUrl=app out=stdout logLevel=4
```
To avoid additional output, one must disable logging with `logLevel=4`.
This method works best with node and rhino, for other environments `console.log` is used.