allow abstract subclasses of SQLAlchemyObjectType

[sjhewitt]

this allows SQLAlchemyObjectType to be subclassed so you can provide common functionality. My use case is to override resolve_id, but this could/should be extended to allow the abstract subclass to provide field declarations and perhaps common meta options (interfaces, etc)

[coveralls]

Coverage remained the same at 90.254% when pulling 5f83153 on sjhewitt:allow-abstract-subclasses into eb4df8d on graphql-python:master.

[syrusakbary]

This is now handled automatically in the upcoming Graphene 2.0. Closing :)

graphql-python/graphene#500