Fixed: url()s to files outside the JSMVC root, were badly converted for production.css

### Overview

When a stylesheet uses a relative url() reference to a file that lies outside/above the JSMVC root, and production.css is built, it will convert the url() to an incorrect path. It might also be necessary to build production.css to a different path.

### Testing

See the updated test case:
`./js steal/build/styles/test/styles_test.js`

### Notes

* The fix was to update `File.joinFrom()` in `steal/rhino/file.js` with the slightly updated `File.joinFrom()` in `steal/steal.js`. However, there's probably a bigger issue here, since both File interfaces have almost identical code -> issues due to violating DRY.
* This is the same issue as described in #81. However, that issue isn't well described, nor is there a test to ensure it works...