Module maintentace

* Fix docstrings now that contact.py is gone.
* Slightly improve code style. Delegate to __eq__
* Simplify exception handling in removeContact. The docstring should be
  enough documentation for anyone reaching this code via a traceback.
* Drop comment mentioning IndexError.
* Store market_id value for future reference.
Improve code coverage of unit tests.

* Test getContacts on empty bucket.
* Test market_id attribute.
* Factor kbucket creation.