Expose Package, Resource, and ResourceReader in the namespace

Closes python#38

Author: warsaw

importlib_resources/__init__.py

@@ -7,7 +7,8 @@
 
 if sys.version_info >= (3,):
     from importlib_resources._py3 import (
-        contents, is_resource, open, path, read)
+        contents, is_resource, open, path, read, Package, Resource)
+    from importlib_resources.abc import ResourceReader
 else:
     from importlib_resources._py2 import (
         contents, is_resource, open, path, read)