mozilla_sync: Implemented missing methods.

Implemented the following APIs:
 * GET https://server/pathname/version/username/info/collection_usage
 * GET https://server/pathname/version/username/info/collection_counts
 * GET https://server/pathname/version/username/info/quota

The specification [1] explains them in more detail.
See also owncloud/apps#384

[1] http://docs.services.mozilla.com/storage/apis-1.1.html#apis