Update `driver.js` to fit with the new JSON encoding of sum types

[kfl]

Update driver.js to fit with the JSON encoding of sum types that Data.Aeson.genericToJSON use. Thus really fixing #71.

[HeinrichApfelmus]

Thanks! However, as mentioned, in this case I prefer to make an explicit ToJSON instance, so that the communication protocol is specified entirely within the Threepenny source code and does not rely on (non-existing) documentation of the aeson library (at least for aeson-0.6).

[kfl]

I agree that making the ToJSON instance explicit is better.

However, I don't think that it is the right JSON encoding, so I have made an updated version that mimics genericToJSON rather than Data.Aeson.Generic. This is done so that we can avoid the for(var key in event) construct in driver.js which is both brittle and somewhat non-deterministic.

[HeinrichApfelmus]

Sounds good to me, thanks!