Nvelope

- `EnumConversion` class

- JSON schema generation in compound objects via `.schema()` method;
- `Conversion` interface now requires `.schema()` method returning a JSON schema;
- moved functionality of `ObjWithAliases` to `Obj`;
- added possibility of storing undefined JSON fields in a model instance;
- `validated` class decorator checking the correctness of a `Obj` and `Arr` subclass;
- `datetime_timestamp_conv` to store a `datetime` as POSIX timestamp float.

In 0.4.0 `NvelopeError` is thrown if something goes wrong. `NvelopeError` contains path to the exact field that caused the error.

Better support for string type annotations