javax.faces.convert - refactor common behaviour + DateTimeConverter changes

All available converters look very similar. Extract the common behavior in base class.

Also DateTimeConverter can be migrated to work with type safe enums for style and type properties.
There are comments in source like //TODO: validate timeStyle. According to java doc of DateTimeConverter on sun there should not have validation. The validation of these will be performed when asString/asObject methods are called.