[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?
I think they're not needed, also confusing for someone who'd read the source. There is also no chance it's gonna be called in code... Please remove or if you think the message is needed - comment out.