Fix addAssetDir bugs

One bug (I think) was the the assetPath argument to walk() was getting stomped
on and therefore not being used, so I removed the argument. Another bug was that
the url for an asset was being set from the file's absolute path.
Remove unused var assetPath
Remove references to assetPathPrefix that I missed
Revert "Fix addAssetDir bugs"

This reverts commit 5a3baa592cf1d4f5b14eddf2ac760886d81a6737.

Conflicts:
	tools/bundler.js
Add test for bundling apps with public/ directories
Fix stomped-on assetPathPrefix in addAssetDir
Revert "Remove references to assetPathPrefix that I missed"

This reverts commit c45952df35ed949bda65b13e3e7fef9a0d9474d5.