Add-Type: add mscorlib.dll to the list of default references

Fix #1361
On CoreCLR 1.0.0 RTM the layout changed compare to RC2.
This change accounts for it.
This change also cleans up some left-overs for SecureString ref.

test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1

@@ -1,5 +1,15 @@
+$guid = [Guid]::NewGuid().ToString().Replace("-","")
+
 Describe "Add-Type" {
     It "Should not throw given a simple class definition" {
-	{ Add-Type -TypeDefinition "public static class foo { }" } | Should Not Throw
+        { Add-Type -TypeDefinition "public static class foo { }" } | Should Not Throw
+    }
+
+    It "Can use System.Management.Automation.CmdletAttribute" {
+        $code = @"
+[System.Management.Automation.Cmdlet("Get", "Thing", ConfirmImpact = System.Management.Automation.ConfirmImpact.High, SupportsPaging = true)]
+public class AttributeTest$guid {}
+"@
+        Add-Type -TypeDefinition $code -PassThru | Should Not Be $null
     }
 }