Cygwin support

As discussed in #506, Cygwin requires *native <-> Cygwin* filename translations.

In this new pull request  default translation functions are provided for Cygwin systems, so users don't have to setup this.

*cygpath* is awailable on every Cygwin system. It's implementation is not trivial. I don't see any advantage reimplementing this in Elisp. It just adds complexity and thus another source of error.