remove Configurable.created ?

[Carreau]

What is the point of Configurable.created ? it does not seem to be use anywhere :

$ ack '\.created' **/*.py
IPython/config/configurable.py 105:        self.created = datetime.datetime.now()
IPython/nbformat/v2/nbbase.py 159:        metadata.created = unicode_type(created)
IPython/nbformat/v2/tests/test_nbbase.py 109:        self.assertEqual(md.created, u'today')
IPython/nbformat/v3/nbbase.py 195:        metadata.created = cast_unicode(created)
IPython/nbformat/v3/tests/test_nbbase.py 139:        self.assertEqual(md.created, u'today')

[minrk]

I didn't know it was there, I would remove it (#4613).