async

This is a quick hack because I needed this, so not really a serious pull request.

But what does the author think about an async version? I wouldn't want to maintain yet another pandoc wrapper ;)

- callbacks all around
- all calls to pandoc use execHandler() function which uses a callback if given

It should still work with the old api, but I haven't tested it fully (just that my old build script still works, sync and async).