Mapper Parent classes

Added common builder interface and mapper classes to hold common method definitions and classes variables. 
Key changes:
* Added BuilderInterface: This is a common interface for all the other builders to extend. Currently it just holds the fix field description method definition that is common across all the interfaces. 
* Added BaseMapper: This is a simple mapper class that currently just holds some common variables and method definitions. Helps remove some repeated code in the specific mapper classes.
* Added BaseGroupedMapper: This abstract class adds the "with" and "end" methods to the BaseMapper making it possible to define grouped field lists. Simplifies the FormMapper and ShowMapper classes a bit