Keep paths in main 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. 

Example: `users/list.js` and `tasks/list.js` would both be bundled to `bundles/list.js`; whichever is built last would overwrite the first.

With this fix, `users/list.js` would build to `bundles/users/list.js`, and `tasks/list.js` would build to `bundles/tasks/list.js`.

There may be other schemes to generate a bundle name but they a) are more complicated and/or b) can't guarantee uniqueness as well as just using the pathname.

**NOTE:** 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. **EDIT:** Added a patch to steal here: https://github.com/bitovi/steal/pull/222

Can you explain how bundles.length === 1 means it's the main bundle?