Add Unit level Test for Domain Conference

* Verify null constructor args not allowed
* Verify null setter args not allowed
* Verify addX null not allowed
* Verify Date state not leaking
Add Unit level Test for Domain Attachment

* Verify null constructor args not allowed
* Verify null setter args not allowed
* Verify addX null not allowed
* Verify Date state not leaking
Add specification for the Timestampable interface

* Verify the behavior of created / modified dates
Fix missplaced unit test