Feature: Add unit-test friendly features

[dclucas]

Add features to help on client code unit tests. For example:

1. Add events to:

• When a property mapping is discovered;
• When an object mapping begins/ends;
• When a property is mapped;
  1. Add auxiliary methods to help determine:
• Which source and target properties have been touched by the mapper;
• Which haven't.

For the time being, this issue is a placeholder for the functionalities we can think of. Once we figure out which ones we want to implement, specific items will be created for them.