Fixes #175 - Staging does not clear previous files.

* Use sync when copying to staging directory
* Add test to make sure incrementally removing files shows up correctly.

NOTE: we may need to feed this through all our staging, e.g. when generating
MSIs or RPMs or DEBs.

Fixes #175  (Not sure why github requires that here as well as title...)


Review by @muuki88 or @aparkinson