Add support for Unions (#91)

* Add support for UnionType, refs #63

* Extract Union functionality into extra class UnionType

* Add documentation for Union Types

Author: jthomaschewski

Readme.md

@@ -83,6 +83,7 @@ config/graphql.php
 - [Pagination](docs/advanced.md#pagination)
 - [Batching](docs/advanced.md#batching)
 - [Enums](docs/advanced.md#enums)
+- [Unions](docs/advanced.md#unions)
 - [Interfaces](docs/advanced.md#interfaces)
 
 ### Schemas