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.
Merge pull request #202 from sbt/wip/fix-175

Fixes #175 - Staging does not clear previous files.