Write doctests for soy.bodies.Billboard

There should be a general doctest in the tp_doc (main class docs) showing generally how the class can be used and one in each method/property showing the basic usage of that method/property. Use the same indentation as the doctests for other classes so this works correctly with Sphinx.

The finished doctests should pass unless you've found something that's broken, in which case submit the test that demonstrates the bug you've discovered.

While working on this task you should join and remain in #PySoy on Freenode to get help, feedback, and guidance from mentors and other developers. Code updates which may affect your work are also announced here as they happen.

When you've done, commit your work and post the resulting changeset url to this task.