fixes shebang modification when osx_is_app is true

The documentation for the conda build framework at http://conda.pydata.org/docs/build.html includes an `osx_is_app` metadata parameter, which instructs conda that python.app should be used instead of python for entry points. The logic exists in post.py to modify the shebang when `osx_is_app` is true, but there is no mechanism to pass a true value to the fix_shebang function.

This pull request provides the needed implementation. It also does some minor reorganization:

1. Pass the entire package metadata instance to `post_build`, and let `post_build` query the parameters it requires.
2. Check `sys.platform` outside of the for loop.
3. Don't call `fix_shebang` from inside `mk_relative`. This was a non-obvious combination of responsibilities. Also, `mk_relative` doesn't use `osx_is_app`, but would have to pass that kwarg on to `fix_shebang`.