Track and Provide Access to Changed Properties

[HC-224]

Tracks an entities changed properties and the values loaded from SQL.

I needed this to implement an auditing system like #135 seems to be asking for, but how that should work varies and there is not a single solution. These changes should provide what is needed for users to implement this system themselves.

I used these changes locally and was able to create an auditing system that tracks entity values changed by ktorm.