Add unstubbed lookup of root

Testing with only stubbed `Etc.getpwnam` makes me nervous; checking for
the name and UID of root should work consistently across platforms.
Implement `getpwuid` lookup function

FIXME: Refactor this and `getpwnam` to be more DRY.
Add example cases for both string and in args
Add example of exception raised due to bad argument
Convert arg[0] to int
Insist on rspec < 3.0.0