- Add func ValidatorEngine for returning the underlying validator engine used
  in the default StructValidator implementation.
- Remove the function RegisterValidation from the StructValidator interface
  which made it immpossible to use a StructValidator implementation without the
  validator.v8 library.
- Update and rename test for registering validation
  Test{RegisterValidation => ValidatorEngine}.
- Update readme and example for registering custom validation.
- Add example for registering struct level validation.
- Add documentation for the following binding funcs/types:
  - Binding interface
  - StructValidator interface
  - Validator instance
  - Binding implementations
  - Default func