[DRAFT, WINDOWS] Use platform-specific separator

Possible support for https://github.com/rspec/rspec-dev/issues/83
I think I'd rather see `def self.separator` instead be local variable assignment like:

``` ruby
sep = File::ALT_SEPARATOR ? "\\\\" : File::SEPARATOR
```

And then this line can be much shorter, using `sep` rather than `self.separator` :).  Also, that should be a bit faster as there's no reason to recalculate the separator on each use.