Add postflight, make scripts operations conditional on file existence

This commit adds support for templatizing a postflight for OS X packages and
also makes both the postflight and preflight optional. If they don't exist the
task doesn't templatize or copy them.
Use PACKAGEMAKER constant

The tool check here uses a hardcoded path for packagemaker when a constant is
defined for it at the start of the task.
Merge pull request #91 from haus/postflight

Postflight