add empty data binding plugin
add data binder spring bean
add getTargetType() to ValueConverter
improve data binder flexibility by allowing converters to injected

Some relevant functional tests:
https://github.com/grails/grails-functional-tests/commit/17cd3af1fd02263dbaf8621d66ed010fc2c28ee6
https://github.com/grails/grails-functional-tests/commit/cff27668c143cf0cd8c59fc5b45bd73686be0504
https://github.com/grails/grails-functional-tests/commit/201e93b9366b52791de57d8fda17a788f24f7f44