The cumulative time of each TestFixture was wrong

The cumulative time of each TestFixture was wrong, considering that should accrue only times of their children fixtures, but each TestFixture was always considering the accumulated time. 

Moreover, the timing system was replaced by TStopWatch to capture more accurate time.

PS: I do not know an appropriate way to create unit tests to this type of change, so the lack of testing in this pull request.

Before fix:
![nunit_results_before_fix](https://f.cloud.github.com/assets/842859/2241547/7c980e92-9cd7-11e3-9578-791417076e17.png)

Fixed:
![nunit_results_fixex](https://f.cloud.github.com/assets/842859/2241549/83199a56-9cd7-11e3-938d-266c87576caf.jpg)