Graphene 0.3.0 - Mutations, Containers and a big refactor

Changelog

Based on this issue #15, improved the following:

• Register types as schema access it.
• Simplify schema base types. Avoid @memoize hacks
• Add field description based on resolver function __doc__
• Add python class enums.Enum mapping to a GraphQL Enum
• Map received arguments keys by resolvers to be snake_case (via ProxySnakeDict)
• Add Mutations
• Add Relay Mutations
• Improve containers: Generalize use of instance/types instantiation
• Improved testing coverage
• Refactored code