decorators for validations

Well, I'm currently working on decorators for aurelia validation.
At the moment, I have the following code working:
```
export class ClientViewModel {
public Id: string;

@required()
public Name: string;

@displayName("Identity")
@email("It's not an valid email")
@satisfiesRule("cpfcnpj")
@required()
public PersonIdentity: string;

constructor(data: any = {}) {
this.Id = data.Id || null;
this.Name = data.Name || null;
this.PersonIdentity = data.PersonIdentity || null;
}
}
```

After I'm done, would you guys be interested in accept a PR?

The issue provides a ClientViewModel example using required, displayName, email, and satisfiesRule decorators, but names no files or tests. Start by reviewing the existing Aurelia validation APIs and how these decorators would fit them. Done would be a clearly scoped implementation accompanied by a pull request, though the issue does not define acceptance criteria.

Written by the indexing model from the issue text.