joala-54, #54: Minor refactorings.
joala-54, #54: Changed contract: removeProperty fails if property is not set.

This should provide more consistency with getProperty().
joala-54, #54: More tests and some more documentation.
joala, #55 Reference: Possibility to check if reference is actually set

Fixed.
joala, #42 References should be able to hold null values

Fixed.