Make Add-Type work without a flat folder structure

- Get rid of the assumption that all framework assemlbies live in
  the same place
- Enable build scenario (assemlbies are referenced directly from unpacked
  nuget packages)
- Fix #766
- Re-enable Add-Type tests

test/powershell/Add-Type.Tests.ps1

@@ -1,5 +1,5 @@
 Describe "Add-Type" {
-    It "Should not throw given a simple class definition" -Skip {
+    It "Should not throw given a simple class definition" {
 	{ Add-Type -TypeDefinition "public static class foo { }" } | Should Not Throw
     }
 }