Reduce Collection's public interface

[keimlink]

The Collection class has too many public attributes and methods which confuse users if they are using a Collection instance.

Attributes that should be private:

• as_dict
• as_list
• element_base
• element_class
• fields
• flat

Methods that should be private:

• get_values

[keimlink]

element_class is actually just the name of the new Element class. So it should be renamed to element_class_name.