Refactor of pynag object creation

Hey, 

This PR only refactors code for pynag object creation.

Here are the important changes:
 * StringToClass is now a Class instead of a dict. It implements a get method so the instantiated string_to_class takes care of the backwards compatibility.
 * Moved all_attributes dict to nagios_attrbutes because they are Nagios specific.
 * Methods and Classes involving Nagios objects have been moved to nagios_objects.py

This opens up the ability to create new classes from base nagios objects or create new classes with new attributes for different backends (Some monitoring solutions introduced slight modifications to Nagios objects. It is now easy to consider these modifications without impacting the default behavior of Nagios objects in pynag)

What are your thoughts?