Notifications-python-client

Added

* Added `NotificationsAPIClient.send_letter_notification()`:
* functions similarly to `send_sms_notification` and `send_email_notification`, however, address details are provided
in the personalisation dict
* See more details in the [README](https://github.com/alphagov/notifications-python-client#letter)

Changed

* Update to `NotificationsAPIClient.get_template(id)`:
* the `dict` returned has changed, learn more from the [README](https://github.com/alphagov/notifications-python-client#get-a-template-by-id).
* Update to `NotificationsAPIClient.get_template_version(id, version)`:
* the `dict` returned has changed, learn more from the [README](https://github.com/alphagov/notifications-python-client#get-a-template-by-id-and-version).
* Update to `NotificationsAPIClient.get_all_templates(template_type=None)`:
* the `dict` returned has changed, learn more from the [README](https://github.com/alphagov/notifications-python-client#get-all-templates)
* Replaced `NotificationsAPIClient.get_template_preview(id)` function with `NotificationsAPIClient.post_template_preview(id, persionalisation)`:
* See more details here [README](https://github.com/alphagov/notifications-python-client#generate-a-preview-template)

Changed

* `NotificationsAPIClient.get_all_notifications()`
* Notifications can be now be filtered by `reference`, see the README for details.
* NotificationList response has changed, see the [README](https://github.com/alphagov/notifications-python-client#get-the-status-of-all-messages-with-pagination) for details.

Changed

* Python 2.7 supported

Changed
* Using version 2 of the notification-api.
* When creating a new NotificationsAPIClient the optional parameter of service_id has been removed. The api_key is a combined string contain the api_key and the service_id
* Update to `NotificationsAPIClient.send_sms_notification()`:
* added `reference`: an optional unique identifier for the notification or an identifier for a batch of notifications. `reference` can be omitted.
* the `dict` returned has changed, learn more from the [README](https://github.com/alphagov/notifications-python-client#text-message).
* Update to `NotificationClient.send_email_notification()`:
* added `reference`: an optional unique identifier for the notification or an identifier for a batch of notifications. `reference` can be omitted.
* the `dict` returned has changed, learn more from the [README](https://github.com/alphagov/notifications-python-client#email).
* Update to the response of `NotificationsAPIClient.get_notification_by_id(id)`, see the [README](https://github.com/alphagov/notifications-python-client#get-the-status-of-one-message) for details.

Changed
* `BaseClient` method signature: `base_url` is now optional. See [41](https://github.com/alphagov/notifications-python-client/pull/41)
* `BaseClient` method signature: `api_key` is now a positional argument. See [41](https://github.com/alphagov/notifications-python-client/pull/41)