delta : make Delta a concrete class and remove subclasses

This is a performance optimization, deltas are a hot code path; this
also aligns better with the js and how we treat other similar objects in
the wrapper layer.
CHANGELOG : add a note regarding delta type change
pubspec : bump version to 0.5.0-dev as the delta API change is potentially breaking
proxy : use the delta action constants in `_jsDelta`
Merge pull request #48 from rmsmith/deltas

Deltas