Automatic deriving of instances for Json

Example:

data GeographicalLocation = GeographicalLocation {
    latitude :: Double,
    longitude :: Maybe Double
}

deriving instance Json GeographicalLocation

testIt :: <Proc> ()
testIt = do
    print $ toJsonString GeographicalLocation {
        latitude = 1,
        longitude = Just 2 }
    print (fromJsonString
        "{ \"latitude\": 1, \"longitude\": 2 }"
        :: GeographicalLocation)
    print $ toJsonString GeographicalLocation {
        latitude = 1,
        longitude = Nothing }
    print (fromJsonString
        "{ \"latitude\": 1 }"
        :: GeographicalLocation)

#256

Change-Id: I7f44b65472436779045e4d15871858d14132dff9