Moved @named_data into main ddt.py module so it can be imported.

[orgadish]

Issue #106

Changes:

• Moved @named_data and NamedData types into main ddt.py module so it can still be imported as a single module.
• Updated tests accordingly.

Testing:

• test_named_data and all non-yaml tests in test_example are successful using Python 2.7 and 3.10

[orgadish]

@wswld This is a simple change. Let me know if it makes sense or if you prefer to resolve #106 a different way.