Add Support for resource References

[hayk99]

It would be great to give support to resource references, for example:

oneUser:
    Type: AWS::IAM::User
    Properties:
      Path: /bot/
      UserName: foobar

  userCredentialsOne:
    Type: AWS::IAM::AccessKey
    Properties:
      Status: Active
      UserName: !Ref oneUser

• As a first iteration, it would be interesting to keep the reference instead of replace the value by nil/null (code line).

• The next iteration could be interesting to return the value of the referenced resource, in this case !Ref oneUser should return oneUser UserName

Thanks !!