Keep paths in bundle names to ensure unique bundle names, fixes #52

Fixes #52.

Main bundle names should use the full path. If we only use the filename, we may conflict with other main bundles, e.g. `users/list.js` and `tasks/list.js` should be `users/list` and `tasks/list`, whereas under the old behavior, both would have the same name `list`.

This also requires a matching change to StealJS' `steal.js` and steal.production.js`. In `addProductionBundles()`:

    mainBundleName = bundlesDir+filename(main);

should become:

    mainBundleName = bundlesDir+main;

I'll submit a PR for that too.