Added method for Connect subscriptions [ch4617]

[hassansin]

Added connectSubscription method: https://dev.chartmogul.com/reference#connect-subscriptions
Example:

chartmogul.Customer.connectSubscriptions(config, uuid='cus_5915ee5a-babd-406b-b8ce-d207133fb4cb', data={
  'subscriptions': [
    {
      "data_source_uuid": "ds_ade45e52-47a4-231a-1ed2-eb6b9e541213",
      "external_id": "d1c0c885-add0-48db-8fa9-0bdf5017d6b0"
    },
    {
      "data_source_uuid": "ds_ade45e52-47a4-231a-1ed2-eb6b9e541213",
      "external_id": "9db5f4a1-1695-44c0-8bd4-de7ce4d0f1d4"
    }
  ]
})