Make xunit runner task take IncludeTraits and ExcludeTraits properties
Add error to build.cmd when running in a non DevEnv command prompt.

Update links to developer guide in error messages to new location
on the dotnet/corefx wiki.
Merge remote-tracking branch 'kr/XunitTraits'
Cause appveyor to run CI build in Dev12 developer command prompt.
Cause build.cmd to set errorlevel to 1 on failed prereqs