[v0.1a4] graphql-core v0.1-alpha-4 release

This release implements the Type Validation System (#8)

• Implement type validation and unit tests #8
• Implement assert_object_implements_interface #8
• Implement missing enum_type tests.
• Removed submodule libgraphqlparser, move everything we need into scripts.
• Make generate_ast work on py3x
• Use slots in classes that seem to be instantiated a lot (to save some memory)