Added support for application factory pattern

flask_mongorest/__init__.py

@@ -1,11 +1,20 @@
-from flask import Blueprint
+from flask import Flask, Blueprint
 from flask_mongorest.methods import Create, BulkUpdate, List
 
 
 class MongoRest(object):
-    def __init__(self, app, **kwargs):
-        self.app = app
+    def __init__(self, app=None, **kwargs):
+        self.app = None
         self.url_prefix = kwargs.pop('url_prefix', '')
+
+        if app is not None:
+            self.init_app(app)
+
+    def init_app(self, app):
+        if not app or not isinstance(app, Flask):
+            raise Exception('Invalid Flask application instance')
+        self.app = app
+
         app.register_blueprint(Blueprint(self.url_prefix, __name__, template_folder='templates'))
 
     def register(self, **kwargs):