Serializer/deserializer overhaul

[jasongrout]

Currently the signature of Javascript serializers is confusing (they are called with a second argument that is different depending on who is calling them). Additionally, we have an instance serialize and a static serialize. #1300 addresses some of this, but we should get a clean consistent picture of serializers and deserializers for 7.0.

I'd like whatever we come up with to address:

1. transferring information by transferring a delta of the existing attribute (which means that the deserializer needs to have access to the current state, which means the deserializer probably can't be called from a static method)
2. Consistent signature for the serializer/deserializer

[jasongrout]

Moving to Future - this is a trickier issue than I think we want to tackle for 7.0.

CC @SylvainCorlay, @maartenbreddels.