NUnit 2.6.6

This is an incremental improvement on NUnit 2.6.5, mostly focused on improving the build, which is now Cake-based. A single new NUnit 3 compatibility warning was added as well.

What's Changed

• Show compatibility error for async void tests by @CharliePoole in #40
• Simplify build scripts by @CharliePoole in #41
• Replace NAnt script with Cake by @CharliePoole in #43
• Use NuGet packages for all external dependencies by @CharliePoole in #45
• Update links and info in nuget packages by @CharliePoole in #47
• Add package for nunit core by @CharliePoole in #48
• Prepare for release by @CharliePoole in #49

Full Changelog: 2.6.5...2.6.6