Add Cygwin support. Refs #735

`cider-from-nrepl-filename-function` and
`cider-to-nrepl-filename-function` are used to translate from/to nrepl
filenames.

Add a default implementation on Cygwin systems utilizing "cygpath". Use
`identity` otherwise.
Merge pull request #735 from juergenhoetzel/cygwin

Cygwin support