Rename index.css and require.js with md5 hash

- removes obsolete "cache buster" setting
- replaces hardcoded require.js and index.css filenames in settings
files for non-dev build processes with variables.
- `release` and `couchapp_deploy` tasks now rename those two files to
include their hashes, and reference them in the generated index.html
file
You can just use node's `fs.unlinkSync()` here - no need to rely on the clean task:

```
fs.unlinkSync(fileChanges.oldPath)
```

http://nodejs.org/api/fs.html#fs_fs_unlinksync_path