hacking around with custom elements

committing so that I don't lose the work, but we can't actually move to
this stuff yet.
pubspec : add `custom_element` to dev_dependencies for sunflower example
rename `registerObjectEditorElement` -> `registerFieldsCustomElements` and guard against double registration
pubspec : update to tags 0.3.0
sunflower.dart : query -> querySelector
showcase.dart : update
update CHANGELOG and README for next version
Merge pull request #37 from rmsmith/custom_elements

Custom elements