Fix for EPERM on windows

A quick and kind of hacky fix for the `EPERM` errors that occur on windows (Most commonly for me involving deleting `.git` folders). This fixes [#21](https://github.com/isaacs/rimraf/issues/21)

I've used `chmodSync` even in the async code, simply to avoid touching the existing structure, but this can be changed if desired. 

`chmod` on windows only supports `444` and `666` which toggle read only.