Implement a JWT header into base client

- adds a base client
- adds a notifications client

These do not proxy onto genuine methods. This pull request is the basic implication of the API Client. Still needs a few things before is ready, notably proper logging and actual API endpoints to hook into.

Basic premise is to deliver the JWT tokens required for Notify API authentication so we can discuss the implementation/premise.

tests/app/main/test_authentication.py

@@ -0,0 +1,82 @@
+from flask import json
+import jwt
+import hashlib
+import hmac
+import calendar
+import time
+import base64
+
+
+def sign_a_thing(thing_to_sign, secret_key):
+    return base64.b64encode(
+        hmac.new(
+            secret_key.encode(),
+            thing_to_sign.encode(),
+            digestmod=hashlib.sha256
+        ).digest()
+    )
+
+
+def token(request_method, request_url, secret_key, service_id, json_body):
+
+    # request method and resource path - hash of request url - POST /path-to-resource
+    signed_url = sign_a_thing("{} {}".format(request_method, request_url), secret_key)
+    signed_payload = sign_a_thing(json_body, secret_key)
+
+    headers = {
+        "typ": "JWT",
+        "alg": "HS256"
+    }
+
+    claims = {
+        'iss': service_id,  # issued by - identified by id of the service
+        'iat': calendar.timegm(time.gmtime()),  # issued at in epoch seconds
+        'req': signed_url.decode(),
+        'pay': signed_payload.decode()  # signed payload
+    }
+
+    return jwt.encode(payload=claims, key=secret_key, headers=headers).decode()
+
+
+def test_should_not_allow_request_with_no_token(notify_api):
+    response = notify_api.test_client().get("/")
+    assert response.status_code == 401
+    data = json.loads(response.get_data())
+    assert data['error'] == 'Unauthorized, authentication token must be provided'
+
+
+def test_should_not_allow_request_with_invalid_token(notify_api):
+    response = notify_api.test_client().get(
+        "/",
+        headers={'Authorization': 'Bearer 1234'}
+    )
+    assert response.status_code == 403
+    data = json.loads(response.get_data())
+    assert data['error'] == 'Forbidden, invalid authentication token provided'
+
+
+def test_should_allow_request_with_valid_token(notify_api):
+
+    body = {
+        "1": "A",
+        "3": "A",
+        "7": "A",
+        "15": "A",
+        "2": "B"
+    }
+    request_body = json.dumps(body)
+
+    request_token = token("POST", "/", "1234", "service1", request_body)
+
+    # from time import sleep
+    # sleep(4)
+
+    response = notify_api.test_client().post(
+        "/",
+        data=request_body,
+        headers={
+            'Authorization': "Bearer {}".format(request_token)
+        },
+        content_type='application/json'
+    )
+    assert response.status_code == 200