changes around fitness_activity

Incorporating jupp0r's changes
https://github.com/jupp0r/health_graph/commit/8023d143524ea187afa79c4895b81e4a209e3d8c
https://github.com/jupp0r/health_graph/commit/e8ee12218fbbe277b1fa0c60bff762f624f28289
https://github.com/jupp0r/health_graph/commit/1eb68a619bda471f7fa737aa52295952cf2ba050
https://github.com/jupp0r/health_graph/commit/6c77b0ee2ebc495d5a17a4c68c621566ab86ac28
https://github.com/jupp0r/health_graph/commit/aac3be6dc4ec870d0dff7445408982e6174e3992

Making fitness_activity available through the user connection. This isn't the way that the API is structured, but it's a bit more convenient for my own uses.