Fix rsync on windows (alternative)

Alternative to PR #67, akin to how it is [solved in mccloud](https://github.com/tknerr/mccloud/commit/56fc78eb0640bc8ac51b85a472ec3bab17ac0989) and [knife-solo](https://github.com/tknerr/knife-solo/commit/21def85d7d8c633ff2f47dd27afbbe88d5085252):

* convert to `/cygdrive/..` style pathnames on windows
* fix permissions by setting `--chmod ugo=rwX` on windows
* warn if no `rsync` available (cherry-picked 114e10e1ec613deffcc85851015e2db748332714 from @databus23) 

See [this comment](https://github.com/mitchellh/vagrant-aws/pull/67#issuecomment-18865716) for more details.