Fix assert message in Register class. Fixes #91.

[geertjanvdk]

Previously, when trying to register an ObjectType which is not supported by Registry-class, an AttributeError is raised instead of AssertError.

We correct the problem by making sure we do not call format() on a tuple.

Tests were added for the Registry.register() method.

[coveralls]

Coverage remained the same at 90.813% when pulling 80f1a6b on geertjanvdk:fix/91 into 136d2b3 on graphql-python:master.