Clear out UnitTestSchedulers when setting InUnitTestRunnerOverride
Remove Pex from UnitTestRunners to check for
Could match WriteableBitmapEx for example
Unbreak the build
Merge remote-tracking branch 'origin/master' into rxui5-master

Conflicts:
	ReactiveUI.Tests/ReactiveUI.Tests_SL5.csproj
	ReactiveUI.Tests/RxAppTest.cs
	ReactiveUI/RxApp.cs