[csharp] Options: useCollection, returnICollection

This change is inline with Microsoft's recommended guidelines for
collects
(https://msdn.microsoft.com/en-us/library/dn169389(v=vs.110).aspx).

Added generator options for csharp to:

* useCollection: Deserialize responses into and return Collection<T>
* returnICollection: For List<T> or Collection<T>, return ICollection<T>
  instead of the concrete type

As a consequence of useCollection, method imputs will also change to
Collection<T>.

samples/client/petstore/csharp/SwaggerClientTest/TestPet.cs

@@ -221,7 +221,7 @@ namespace SwaggerClientTest.TestPet
 		{
 		    Assembly _assembly = Assembly.GetExecutingAssembly();
 		    Stream _imageStream = _assembly.GetManifestResourceStream("SwaggerClientTest.swagger-logo.png");
-            PetApi petApi = new PetApi ();			
+            PetApi petApi = new PetApi ();
 			// test file upload with form parameters
 			petApi.UploadFile(petId, "new form name", _imageStream);