Fix introspection and GraphiQL

[Totktonada]

It becomes broken after PR #168. There are two reasons for that:

1. Type names with . and $. Can be fixed in GraphiQL or we can choose another naming convention.
2. Non-standard types InputMap and InputUnion. We can inherit it from Scalar, maybe. InputObject has mandatory fields field, but these new types has no such field.

We also need to add introspection test: ask the same query as graphiql does and check some data are returned.