add `deviceId` and `source` properties to Dexcom data

[jebeck]

Eventually we probably want to provide user access to info about where a particular datum came from, especially for users of multiple CGM systems.

[nicolashery]

I think that's a great idea.

Forgive me for pointing to Nutshell, but maybe this could be useful inspiration? I had used a source attribute on each datum, which was basically an object itself where I could put stuff in that I knew I might need later:

{
  // ...
  source: {
    file: {
      url: "http://url/to/CareLink-Export-1234.csv",
      name: "CareLink-Export-1234.csv"
    },
    device: {
      type: "pump",
      name: "Paradigm Revel - 523",
      id: "342767"
    },
    parser: {
      name: "medtronic",
      version: "0.1.0"
    },
    syncTask: {
      type: "medtronic",
      _id: "52405e19bb59b95f5502406c"
    }
  }
}

See: https://github.com/tidepool-org/nutshell#readings