Client API refactoring

Addressing some suggestions given by Paolo and some other things:

Backwards-compatible changes:
- [ ] Make sure that all places that take a `ValueCoder` also have an overload which uses utf8-strings as default encoding.
- [ ] Add some more default `ValueCoder`s for Integers, Enums, Id-Types (classes wrapping a string).
- [ ] Document the methods that can throw an `AntidoteException`.

Mostly compatible:
- [ ] Refactor the code, so that methods from internal interfaces are not visible in the public interface.
- [ ] Let `CrdtMap` implement `java.util.Map`

Breaking changes:
- [ ] Rename `noTransaction` to `automaticTransaction`
- [ ] Remove subtype-relation between `ObjectRef` and `ResponseDecoder` (this should get rid of some unnecessary methods on `RegisterRef`, `MapRef` and so on)