Support running cf-promises on MacOS X using GNU-find (from homebrew)

When writing policies on Mac OS X it was not possible to run
`cf-promises` due to the GNU-find dependeny.

This commit adds support for "darwin" and assumes that the GNU
`findutils` are installed via [homebrew](http://brew.sh/) to
`/usr/local/`.
Merge pull request #53 from tom-ee/macosx-support

Fixes #4839: Support running cf-promises on MacOS X using GNU-find (from homebrew)