Add Battery storage data

[gtdiehl]

Adding Battery storage data that is coming from /production.json

"storage": [{"type": "acb","activeCount": 4,"readingTime": 1595203104,"wNow": -178,"whNow": 285,"state": "charging","percentFull": 6}

Battery storage data will only be returned when the battery is installed in the Envoy system. To determine if the battery is installed we check for the presence of the percentFull attribute, as in the above example.

If a battery is not present the /production.json page will return:

"storage":[{"type":"acb","activeCount":0,"readingTime":0,"wNow":0,"whNow":0,"state":"idle"}]}

In which case the API will return a message saying the battery storage is not available.

Pull request recommendations:

• [ ] Name your pull request your-development-type/short-description. Ex: feature/read-tiff-files
• [x] Link to any relevant issue in the PR description. Fixes #36
• [ ] Provide context of changes.
• [ ] Provide relevant tests for your feature or bug fix.
• [ ] Provide or update documentation for any feature added by your pull request.

Thanks for contributing!