Added a stopwatch class to the Compatibility namespace along with unit tests. On systems that support the System.Diagnostics.Stopwatch, it chains to that class. On systems that do not support Stopwatch, it is implemented with DateTime Ticks.
Changed the usage of System.Diagnostics.Stopwatch and DateTime in DelayedConstraint to use the NUnit version of Stopwatch.
Visual Studio 2013 insists on adding these changes to the project files to identify them as NUnit test projects. Microsoft does this for performance reasons, see https://connect.microsoft.com/VisualStudio/feedback/details/800245/vs2013rc-adds-to-vs2012-c-project-section-itemgroup.

I am sick of ignoring them, so want to get them locally into the repository.
Increased the sleep in the stopwatch tests to a value that is hopefully above the resolution of the Thread.Sleep call.
Cut 20 seconds off the overall test run (on my machine) by reducing the sleeps and delays in DelayedConstraintTests.
Switch other usages of stopwatch over to the NUnit version.
Merge pull request #172 from nunit/issue-155

Ongoing changes for Issue 155