Name conflict if tracked model has "instance" field

[shemigon]

Problem

If a tracked model has the instance field, creating a instance of the model throws

TypeError: Model.__init__() got multiple values for argument 'instance'

See issue #633

Solution

Removed instance argument from the new model init method and used args[0] instead.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).