fix(extras): Use uname -r output for extras package name by default

- Optionally, allow a cookbook user to pick the older method of finding
  a kernel, in case the new way breaks compatibilty for them
fix(aufs): Check that the package exists before installing

- Also for new method, use ohai's provided kernel release rather than
  shelling out to uname
fix(aufs): Use newer package finder only on saucy or newer
Merge pull request #35 from jayofdoom/jay/OptionalFindExtrasPackage

Use kernel release for package name on saucy and newer