[FxOS] Support for resolve URI, request all paths and local app directory.

Updated file plugin in preparation for file-transfer plugin implementation. Main change is support for `resolveLocalFileSystemURL` which is needed for file-transfer. I'm also better isolating the filesystems. We now support 3 data paths:
 * `applicationDirectory` - Uses `xhr` to get local files that are packaged with the app.
 * `dataDirectory` - For persistent app-specific data files.
 * `cacheDirectory` - Cached files that should survive app restarts (Apps should not rely on the OS to delete files in here).

### Technical info

To figure out which filesystem to use, file plugin calls the `__format__` function defined in `www/firefoxos/FileSystem.js`, where you have a chance to alter the path sent to the plugin. Over there we add the filesystem name to the full path. Later, the function `resolveToFullPath_` will get a modified full path and return the storage path to be used in indexeddb, the full path of the entry and the file name.

@zalun, can you take a look?
No, they'll all default to null. I kept them here more for documenting what's available. I can remove if you prefer.