Wrap Data.Aeson.Value in a newtype SafeValue

[mchaver]

https://github.com/Vlix/safe-json/issues/20

This isn't complete but I wanted to make sure it is going in the right direction before completing it. Things to note:

1. safeTo creates a SafeValue. I wasn't sure if safeFrom should take Value or SafeValue.
2. I think the SafeValue constructor should not be exported. Just the type and the unSafeValue field.