Notifications-python-client

* Fix issues when installing with pip 10.0.0

* Added post_precompiled_letter_response and updated integration tests
* The API now only returns notification id and client reference, so updated the response scheme to match

* 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.