Indico checkin webhook plugin

This plugin plugin listens for the `registration_checkin_updated`` signal and sends data plus possibly a ticket/badge pdf to a listening webhook.

The data POSTed to the webhook are either multipart form-encoded data with the selected ticket template in files[file] and the registration data json-encoded in form[data] or the json-encoded registration data.

Data layout:

{u'amount_paid': 0,
    u'checked_in': True,
    u'checkin_date': u'<timestamp>',
    u'checkin_secret': u'<uuid>',
    u'data_by_id': {u'<field_id>': u'<value>',
                     ....
  },
    u'data_by_name': {u'<slugified section>_<slugified field title>': u'<value>',
                     ....
  },
    u'event_id': <id>,
    u'full_name': u' Bjoern Pedersen',
    u'paid': False,
    u'personal_data': {u'<field name>': u'<value>',
                       .... personal data fields
  }
  u'registrant_id': u'<registrant id>',
  u'registration_date': u'<timestamp value>'}

(slugified: lowercased, spaces replace by '-', see `indico.util.string.slugify`)