Refactor how strip_root argument works for Codebase #34

A proposed change in scancode.io is that the root Resource of a project codebase will be named . and all other Resources of that codebase will not have the root path prefix in its path. We should change how the strip_root argument for Codebase() works where the root resource is renamed to . and the path set to ., and the root path prefixes are removed from the remaining Resource paths.