Add validation to the PropertyMapper
Add validation functionality to the Property Mapper.
First we need a way to set the validator - a method like setValidator() should be added to the property mapper.
If no validator was set but map() was called, the Property Mapper should try to find one by asking the T3_Framework_Validation_ValidatorResolver (see #70). If none was found, no validation is done.
While mapping we should probably validate each property before setting it in the target object. An open question is if it makes sense to validate the whole target object after mapping.
Validation Errors should be added to the Mapping Results.