Test for XML documentation errors

- extend the "test" target in our build to use new k-xml-docs-test.shade command

Example output with one syntax error and one invalid reference:
```
info: Target xml-docs-test
warn: Invalid documentation syntax in src\Microsoft.AspNet.Mvc.Core\bin\debug\net45\Microsoft.AspNet.Mvc.Core.xml
warn:   3170: <!-- Badly formed XML comment ignored for member "T:Microsoft.AspNet.Mvc.Rendering.HtmlHelper" -->
warn:   3203: If the object is already an <see cref="!:IDictionaries&lt;string, object&gt;"/> instance, then it is
```

makefile.shade

@@ -5,3 +5,6 @@ var AUTHORS='Microsoft Open Technologies, Inc.'
 
 use-standard-lifecycle
 k-standard-goals
+
+#xml-docs-test target='test'
+  k-xml-docs-test