Suap-notifica-govbr

* Add support for document uploads in `send_email_notification`

* 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)