API for dashboards

[justinvdm]

We need to expose an API for the following:

• Creating of dashboards from json config objects (Possibly using POST)
• Removing/clearing of configured dashboards (Possibly using DELETE)
• Getting json data about the dashboard and its widgets (Possibly using GET)
• Checking to see whether a dashboard exists (Possibly using HEAD)

[hodgestar]

:+1: other than the suggestion to move http_request to Agent (although happy for that to be a separate PR).