Support for storing historical runs

[alahiff]

Need to be able to manually specify creation, start and end times. To begin with:

• Run.init() needs additional options created, started and endtime (or similar). If not set, the server will set them (as it does now).
• Run.close() needs an additional timestamp option for specifying the end time. If not set, the server will set it (as it does now).

This doesn't take into account people using the context manager.

When creating a run the JSON body in the POST can include created and started. If there are no metrics then it can also include endtime and status set to completed. When updating the status of the run to completed the JSON body can include endtime.