Add Callback and ScriptCallback
fix windows
add functional include
Merge branch 'callback' of https://github.com/voxelinc/voxellancer into callback
added callbackimpl and scriptcallbackimpl
Merge branch 'master' of github.com:voxelinc/voxellancer into callback
incorporated requests from review
Merge pull request #548 from voxelinc/callback

Add Callback and ScriptCallback