Added SiteSummary class and made site_listing() use it.
Added EngineSummary class and made engine_listing() use it.
Changed engine and site listing method names.

The engine and site listing method names now conform to the official Nexpose API naming convention.
Moved the request() method out of the Connection class.

This will allow for future restructuring to add in more classes for things
like 'Site' object classes and methods under those classes (especially the load() method).
This is part of bringing the code into line with the official Nexpose API conventions.