Memento: PropertyState detection

[mauroservienti]

Connects to #236

This is a first implementation of #236 and allows the following behavior:

var person = new Person();

var memento = new ChangeTrackingService();
memento.Attach( person );

person.FirstName = "Mauro"

var propertyState = memento.GetPropertyState( person, p=>p.FirstName );

At this point propertyState is:

• propertyState.IsChanged -> true
• propertyState.IsValueChanged -> true

Design question:

The ChangeTrackingService has a GetEntityState operation that returns the overall entity state as enum should PropertyState be as well an enum? (I believe so)

Behavior question:

In the above sample .IsValueChanged is true because the default value of the FirstName property is null

Is this the expected behavior?

Obviously the following behaves as expected:

var person = new Person( "Mauro" );

var memento = new ChangeTrackingService();
memento.Attach( person );

person.FirstName = "Giovanni"
person.FirstName = "Mauro"

var propertyState = memento.GetPropertyState( person, p=>p.FirstName );

• propertyState.IsChanged -> true
• propertyState.IsValueChanged -> false

[mauroservienti]

• Changed the return value of ChangeTrackingService.GetPropertyState( ... ) to be an enum to align with GetEntityState( ... ) API
• Added a IsPropertyValueChanged extension method to IMemento entities to simplify usage
• Added public API to IChangeTrackingService interface

var person = new Person( "Mauro" );

var memento = new ChangeTrackingService();
memento.Attach( person );

person.FirstName = "Giovanni"
person.FirstName = "Mauro"

var isFirstNameChanged = person.IsPropertyValueChanged(p => p.FirstName);

@micdenny @nazarenomanco this is now ready for a full review