Make `answerParameters` be a @property of `Distributor`s.

 * ADD `_answerParameters` attribute to the
   `bridgedb.distribute.IDistribute` interface specification.

 * ADD `answerParameters` callable (in implementations, this should
   really be a `@property`) to the `bridgedb.distribute.IDistribute`
   interface specification.

 * ADD `_answerParameters` attribute to
   `bridgedb.distribute.Distributor`.

 * ADD `@property(answerParameters)` methods to
   `bridgedb.distribute.Distributor`.
Add missing logging import to bridgedb.distribute module.