Role/permission initialisation should also accept function

- **I'm submitting a ...**
  [ ] bug report
  [x] feature request
  [ ] question about the decisions made in the repository
  [ ] question about how to use this project

- **Summary**
  Should be able to pass an async function to the constructor, eg: `new ACL(user.getAuth)`
  This would allow for a cleaner flow when fetchin lists of permissions from the server.
