Add support for fields with arguments but without includes

[MurzNN]

I implemented a fix that allows using fields with arguments, but without includes (sub-fields), to fix the issue #31.

An example of queries without sub-fields can be found in the Commercetools GraphQL API here: https://docs.commercetools.com/api/graphql

                masterData {
                  current {
                    slug(locale: "en")
                  }
                }

Also, I added the composer test command to simplify running tests.

Please review it and tell me if something needs to be reworked.