Update readme with proposed new api

- [x] Move validation to `call` => https://github.com/Factlink/pavlov/pull/29
- [x] Move authorization to `call` => https://github.com/Factlink/pavlov/pull/29
- [x] Use Virtus for attributes. => https://github.com/Factlink/pavlov/pull/28
- [x] Fail validation if virtus attribute without default is not given => https://github.com/Factlink/pavlov/pull/30
- [ ] Write README section on validations
- [ ] Add helpers to return both operation itself, and operation result
- [ ] Rename pavlov_options to context and introduce Real Object for it