calling iteritems causes an exception on Python 3

[exhuma]

Example traceback:

Traceback (most recent call last):
  File "lost_tracker/main.py", line 63, in <module>
    social = Social(app, SQLAlchemyConnectionDatastore(mdl.DB, mdl.Connection))
  File "/home/exhuma/workspace/lost.lu/lost-tracker/env/lib/python3.4/site-packages/flask_social/core.py", line 106, in __init__
    self._state = self.init_app(app, datastore)
  File "/home/exhuma/workspace/lost.lu/lost-tracker/env/lib/python3.4/site-packages/flask_social/core.py", line 130, in init_app
    config = update_recursive(module.config, config)
  File "/home/exhuma/workspace/lost.lu/lost-tracker/env/lib/python3.4/site-packages/flask_social/utils.py", line 71, in update_recursive
    for k, v in u.iteritems():
AttributeError: 'dict' object has no attribute 'iteritems'

[exhuma]

Related to #71

[italomaia]

Related: #83