Added ModelMetadata.Container property and unit tests

Fix for #298 

@dougbu, @pranavkm - Could you please help review the code?
instead of adding this test, extend the `MetadataModifierData` data set to include the `Container` property.  test setting property to `null` and one or two non-`null` instances.  for the instances, use some `Type` with a well-understood `Equals`.