v0.8.0

This release brings several minor breaking changes. Please refer to UPGRADE document for details.

Changelog:

• Support for null value (as required by latest GraphQL spec)
• Shorthand definitions for field and argument types (#47)
• path entry in errors produced by resolvers for better debugging
• resolveType for interface/union is now allowed to return string name of type
• Ability to omit name when extending type class (vs defining inline)
• Spec compliance improvements
• New docs and examples