https://github.com/ademchenko/DependenciesTracker/issues/8

- added move implementation (untested)
https://github.com/ademchenko/DependenciesTracker/issues/2

Cover DependenciesTracker by tests
 - added  test for collection property items changing
https://github.com/ademchenko/DependenciesTracker/issues/2

Cover DependenciesTracker by tests
 - added  test for collection changing

https://github.com/ademchenko/DependenciesTracker/issues/9

Adding null item to an observable collection causes ArgumentNullException to be raised.
- added reproducing test
https://github.com/ademchenko/DependenciesTracker/issues/9

Adding null item to an observable collection causes ArgumentNullException to be raised.
- quick fixed (code is not quite good consistent with another parts of tracker)