Backend tests

All serialisers and views currently have 100% coverage. Closes #50 (closed). As every bit of code needs tests a general test issue is not required. Code that wants to be merged into master should be completely tested anyway.

backend/core/urls.py

 from django.conf.urls import url
 from django.contrib.staticfiles.urls import staticfiles_urlpatterns
-from rest_framework_jwt.views import obtain_jwt_token
+from rest_framework_jwt.views import obtain_jwt_token, refresh_jwt_token
 
 from core import views
 
@@ -9,7 +9,8 @@ urlpatterns = [
     url(r'^api/student/submission/(?P<pk>[0-9]+)$', views.SubmissionApiView.as_view()),
     url(r'^api/student/submission/(?P<pk>[0-9]+)/feedback/$', views.FeedbackApiView.as_view()),
 
-    url(r'^api-token-auth/', obtain_jwt_token)
+    url(r'^api-token-auth/', obtain_jwt_token),
+    url(r'api-token-refresh', refresh_jwt_token),
 ]
 
 urlpatterns += staticfiles_urlpatterns()