Remove the JSON instances for (Entity a) and generate them separately in mkPersist

This is related to #180. In this patch I implemented [what I commented in that issue](https://github.com/yesodweb/persistent/issues/180#issuecomment-27550268).

If `mpsEntityJSON` in `MkPersistSettings` is `True`, `mkPerisit` creates `ToJSON` and `FromJSON` instances for each `Entity SomeModelType`. Otherwise, it doesn't create any JSON instances for them.

I didn't change any JSON format and `mpsEntityJSON` is enabled by default, so this change doesn't break existing code.