Add StatLight NuGet package
Add XUnitContrib and StatLight
Change Silverlight5 test to use StatLight
Cleanup Silverlight test file
Upgrade solutions to support VS2013
Use XUnit in SL4 tests
Add SL test assemblies to external
Correct references in SL4 test
Remove unnecessary reference from SL5 test
Delete unused SL testing files

SL2 and SL3 are no longer supported, so there are no need for saving these
files
Remove usage of old SL test runner
Delete old SL test runner