Notifications-python-client

* Added `NotificationsAPIClient.send_precompiled_letter_notification()`
* The client can now send PDF files which conform to the Notify printing template
* Send a File object
* 'reference' must be provided to identify the document

* Use Flake 8 rather than Pep 8

* Fixed PyJWT security issue. [91](https://github.com/alphagov/notifications-python-client/pull/91)

* Added `NotificationsAPIClient.get_received_texts()`
* an optional `older_than` argument can be specified to retrieve all received texts older than the last
received text id. If omitted the first 250, the api limit, received texts will be returned, with links indicating how to retrieve the next page of results.
* Added `NotificationsAPIClient.get_received_texts_iterator()`
* returns all received texts by a generator object.

Changed

* Update to `NotificationsAPIClient.send_sms_notification()`
* added `sms_sender_id`: an optional `sms_sender_id` specified when adding SMS senders under service settings. If this is not provided, the SMS sender will be the service default SMS sender. `sms_sender_id` can be omitted.

* Update to `NotificationsAPIClient.send_email_notification()`
* added `email_reply_to_id`: an optional email_reply_to_id specified when adding Email reply to addresses under service settings, if this is not provided the reply to email will be the service default reply to email. `email_reply_to_id` can be omitted.