Improving MarketoAPI::Campaigns#request behavior.
Fixing a bug with NAMED_KEYS.

- Also removing the Lead constructor function; it doesn't provide a
  sufficiently useful value over Lead.new.
Working around private constants.

- Providing specialized MObject constructor methods on
  MarketoAPI::MObject instead of MarketoAPI.
- Removed the MarketoAPI.MObject constructor function as not
  sufficiently useful over MarketoAPI::MObject.new.
Completing list implementation and documentation.
More documentation updates.