Fix for emitted JSON to be compliant with HomeKit protocol

[geekman]

These changes makes sure that characteristics comply with the HomeKit protocol when a characteristic's ValueRequestFunc returns an error. This would fix issue #30.

Changes:

• GET /accessories must produce a valid "value" in the JSON, even if an error is returned
• a Multi-Status 207 Response must explicitly contain "status": 0 for successful reads, when there are mixed statuses

One thing to note that deviates from the preliminary patch in #30 is that the Value must be tri-state to allow for it to:

• contains a value
• be omitted completely
• and also to be null

So I re-used a pattern that was already in the code, using the V struct. A null value is required because the ProgrammableSwitchEvent needs to read as null, and the test was failing. I also realized that /identify must not emit a value at all.

Also added tests for the changes.

[brutella]

Your changes look fine to me. Thanks