registry: export DataSource class

Expose the juggler's DataSource constructor as `loopback.DataSource`.

The DataSource constructor is most useful to check
for `instanceof DataSource`, but it also makes the loopback API more
consistent, since the API is already exposing all pre-built Models.

lib/registry.js

@@ -318,6 +318,8 @@ registry.autoAttachModel = function(ModelCtor) {
   }
 };
 
+registry.DataSource = DataSource;
+
 /*
  * Core models
  * @private