UseDestinationValue not regarded

Regarding Issue #356 - I am also interested in solving this.

We found that the flag PropertyMap.UseDestinationValue is currently not used. By default is regarded to be on. 
We made three changes: 
* set the initial value of the UseDestinationValue to true (to reflect the current state and dont break anything)
* regard its value in PropertyMap.GetDestinationValue (return null if false)
* added a function in IMappingExpression to disable it
That way we preserve the current behaviour but introduce a way to go back to the old behaviour.

(The change in TypeMapObjectMapperRegistry, seemed a nessesary fix to a hidden bug, revealed by our change. UseDestinationValue does not need to be regarded when setting the value).

Please consider this fix. 
Regards,
Jonas