Temporary fix for Gio.File.prototype

Issue #189 (closed) details the current dilemma of being unable to add or monkeypatch methods on a GObject interface. At this time developers must add an additional line of code to their applications to use Gio.File.prototype. This is a temporary fix faking it until such a fix has been made for that issue.

Example:

promisify(Gio._FilePrototype, load_contents_async, load_contents_finish);