Adapt code to Graphene 2.0

[syrusakbary]

Adapt code to Graphene 2.0

Related PR: graphql-python/graphene#500

[coveralls]

Coverage increased (+0.06%) to 91.706% when pulling c869f7f on 2.0 into 70479ee on master.

[syrusakbary]

@ekampf I've updated the code and refactored the registry code to make it easier to understand and more testable (in a similar way that graphene-django and graphene-sqlalchemy work).

Please review and let me know if you have any suggestion before merging!

[coveralls]

Coverage increased (+0.06%) to 91.706% when pulling 8e4158b on 2.0 into 70479ee on master.

[coveralls]

Coverage increased (+0.06%) to 91.706% when pulling fc520cf on 2.0 into 70479ee on master.

[ekampf]

@syrusakbary thanks! I will go over this over the weekend...

[coveralls]

Coverage increased (+0.06%) to 91.706% when pulling f9f21d4 on 2.0 into 70479ee on master.

[coveralls]

Coverage increased (+0.06%) to 91.706% when pulling e877bc9 on 2.0 into 70479ee on master.

[syrusakbary]

Graphene 2.0 is now merged into master.
As I want to give some people to adapt before releasing the stable version I will merge this PR.

@ekampf please let me know if you have any suggestions by creating a PR (If you want me to take a look) or by pushing directly into master in this repo :)